board: efr32_slwstk6061a enable SoC DC-DC converter

This commit enables internal SoC DC-DC converter on efr32_slwstk6061a
board.

RF components on efr32_slwstk6061a board are connected to the internal
SoC DC-DC converter. Upon startup the DC-DC converter is configured in
the bypass mode. Such configuration provides enough power for the SoC to
boot and perform basic operation. It is not enough however to operate
the radio subsystem. Without this patch enabling radio in Rx or Tx mode
causes voltage drop and triggers brown out detector reset.

Signed-off-by: Piotr Mienkowski <piotr.mienkowski@gmail.com>