boards: arduino_101: Set sensible default for SPI_0_CS_GPIO

The SPI_0_CS_GPIO and SPI_0_CS_GPIO_PIN values were defaulting to
something not very usable, because of which e.g. the file system
test app was having to explicitly set the right values in its sample
configuration. Having a proper defaults in the board defconfig means
this isn't needed anymore.

Change-Id: I1399914451c1616588322e25304d40d3dd1151e7
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>