defconfig: k2g_evm_defconfig: enable SPI driver model

Enable SPI and SPI Flash driver model as K2G SPI controller driver
supports driver model.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Jagan Teki <jteki@openedev.com>