flash: New driver for XMC4xxx microcontroller family

This is a complete flash driver for the Infineon XMC4xxx family of
microcontrollers, based on the TMS570 driver by Andrey Yurovsky.
The driver attempts to discover the particular variant of MCU via a
combination of the SCU register (to determine if this is indeed an
XMC4xxx part) and the FLASH0_ID register (to determine the variant).
If this fails, the driver will not load.
The driver has been added to the README and documentation.

Tests:
* Hardware: XMC4500 (XMC4500_relax), XMC4200 (XMC4200 enterprise)
* SWD + JTAG
* Binary: 144k, 1M

Note:
* Flash protect only partly tested. These parts only allow the flash
  protection registers (UCB) to be written 4 times total, and my devkits
  have run out of uses (more on the way)

Future Work:
* User 1/2(permalock) locking support via custom command
* In-memory flash loader bootstrap (flashing is rather slow...)

Change-Id: I1d3345d5255d8de8dc4175cf987eb4a037a8cf7f
Signed-off-by: Jeff Ciesielski <jeffciesielski@gmail.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/2488
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>

contrib/loaders/erase_check/armv7m_0_erase_check.s

@@ -0,0 +1,45 @@
+/***************************************************************************
+ *   Copyright (C) 2014 by Jeff Ciesielski                                 *
+ *   jeffciesielski@gmail.com                                              *
+ *                                                                         *
+ *   Based on the armv7m erase checker by:                                 *
+ *   Copyright (C) 2010 by Spencer Oliver                                  *
+ *   spen@spen-soft.co.uk                                                  *
+ *                                                                         *
+ *   This program is free software; you can redistribute it and/or modify  *
+ *   it under the terms of the GNU General Public License as published by  *
+ *   the Free Software Foundation; either version 2 of the License, or     *
+ *   (at your option) any later version.                                   *
+ *                                                                         *
+ *   This program is distributed in the hope that it will be useful,       *
+ *   but WITHOUT ANY WARRANTY; without even the implied warranty of        *
+ *   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the         *
+ *   GNU General Public License for more details.                          *
+ *                                                                         *
+ ***************************************************************************/
+
+/*
+	parameters:
+	r0 - address in
+	r1 - byte count
+	r2 - mask - result out
+*/
+
+	.text
+	.syntax unified
+	.cpu cortex-m0
+	.thumb
+	.thumb_func
+
+	.align	2
+
+loop:
+	ldrb	r3, [r0]
+	adds	r0, #1
+	orrs	r2, r2, r3
+	subs	r1, r1, #1
+	bne		loop
+end:
+	bkpt	#0
+
+	.end