cros: exynos: add SPI support for cros_ec

This patch adds SPI support for carrying out the cros_ec protocol.

Signed-off-by: Hung-ying Tyan <tyanh@chromium.org>
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Simon Glass <sjg@chromium.org>