board/efm32: Disable SRST

board/efm32: Disable SRST

The current configuration leads to the following error when trying to
program the target:

  SWD IDCODE 0x2ba01477
  timed out while waiting for target halted
  TARGET: efm32.cpu - Not halted
  in procedure 'program'
  in procedure 'reset' called at file "embedded:startup.tcl", line 478
  in procedure 'ocd_bouncer'

Use the default reset handling of the target (SYSRESETREQ) to reset the
system rather than SRST to fix the problem.

Tested on EFM32GG, EFM32TG and EZR32WG STK.

Change-Id: I788c41baf08b20814cbe0934b563424c4bc144b8
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3420
Tested-by: jenkins
Reviewed-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>