jtag_interface: .speed can be NULL when not needed

adapter_init (core.c) won't check speed configuration
of the selected interface if it's not needed (.speed = NULL).

When it's not needed, we can now omit adapter_khz in
init scripts and we don't have to implement dummy handlers
for speed_div and khz functions.

It also removes calls to adapter_khz in interface configuration
files when not used anymore.

Change-Id: I6eb1894385503fede542a368f297cec6565eed44
Signed-off-by: Franck Jullien <franck.jullien@gmail.com>
Reviewed-on: http://openocd.zylin.com/1131
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>

tcl/interface/sysfsgpio-raspberrypi.cfg

@@ -10,9 +10,6 @@
 
 interface sysfsgpio
 
-# This has no effect on the driver, but is required
-adapter_khz 100
-
 # Each of the JTAG lines need a gpio number set: tck tms tdi tdo
 # Header pin numbers: 23 22 19 21
 sysfsgpio_jtag_nums 11 25 10 9