configs: sama5d27_wlsom1_ek_mmc: add default bootargs

Add default bootarguments with booting from mmc0 for this configuration.
This will allow a default Linux boot for this board.

Suggested-by: Jarvis Chen <jarvis.chen@microchip.com>
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>