flash: New driver for XMC4xxx microcontroller family

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>