boards: arduino_101: enable GPIO by default

Though the SPI_CS_GPIO Kconfig entry (in drivers/spi/Kconfig) has
"select GPIO" specified, we are observing that merely adding
the symbol(SPI_CS_GPIO) in the
defconfig  (boards/x86/arduino_101/Kconfig.defconfig)
is triggering unmet direct dependency warnings
(though the build goes through).

Since the defconfig entry(SPI_CS_GPIO) is not selecting
the aforementioned 'select' rule, we add it  manually here.

Jira: ZEP-1668

Change-Id: Ida6a0c851462d747e6559bd0c78fa52e1d0f24b5
Signed-off-by: Jithu Joseph <jithu.joseph@intel.com>