cros: add cros_ec driver

cros: add cros_ec driver

This patch adds the cros_ec driver that implements the protocol for
communicating with Google's ChromeOS embedded controller.

Signed-off-by: Bernie Thompson <[email protected]>
Signed-off-by: Bill Richardson <[email protected]>
Signed-off-by: Che-Liang Chiou <[email protected]>
Signed-off-by: Doug Anderson <[email protected]>
Signed-off-by: Gabe Black <[email protected]>
Signed-off-by: Hung-ying Tyan <[email protected]>
Signed-off-by: Louis Yung-Chieh Lo <[email protected]>
Signed-off-by: Randall Spangler <[email protected]>
Signed-off-by: Sean Paul <[email protected]>
Signed-off-by: Simon Glass <[email protected]>
Signed-off-by: Vincent Palatin <[email protected]>
Acked-by: Simon Glass <[email protected]>
Tested-by: Simon Glass <[email protected]>