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>

doc/openocd.texi

@@ -5777,6 +5777,22 @@ the flash clock.
 @end deffn
 @end deffn
 
+@deffn {Flash Driver} xmc4xxx
+All members of the XMC4xxx microcontroller family from Infineon.
+This driver does not require the chip and bus width to be specified.
+
+Some xmc4xxx-specific commands are defined:
+
+@deffn Command {xmc4xxx flash_password} bank_id passwd1 passwd2
+Saves flash protection passwords which are used to lock the user flash
+@end deffn
+
+@deffn Command {xmc4xxx flash_unprotect} bank_id user_level[0-1]
+Removes Flash write protection from the selected user bank
+@end deffn
+
+@end deffn
+
 @section NAND Flash Commands
 @cindex NAND