xmc4500: Disable SRST for SDRAM App Kit

xmc4500: Disable SRST for SDRAM App Kit

Just like observed with the General App Kit earlier, it now started to
fail halting:

  SWD IDCODE 0x2ba01477
  TARGET: xmc4500.cpu - Not halted
  in procedure 'reset'
  in procedure 'ocd_bouncer'

  SWD IDCODE 0x2ba01477
  Halt timed out, wake up GDB.

Rely on the target's default sysresetreq behavior to allow flashing to
work seemlessly again.

Change-Id: Ib9ce5f2c0ab99dca6d0fc74435fe26a58437fae5
Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-on: http://openocd.zylin.com/3416
Tested-by: jenkins
Reviewed-by: Jeff Ciesielski <jeffciesielski@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>