drivers: spi: add nRF5 slave driver

SPIS driver for nRF51 and nRF52 SoCs.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
Signed-off-by: Michael Scott <michael.scott@linaro.org>
Signed-off-by: Tyler Baker <tyler.baker@linaro.org>
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>