cros_ec: spi: Add support for EC protocol version 3

Protocol version 3 will be attempted first; if the EC doesn't support
it, u-boot will fall back to the old protocol version (2).

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Randall Spangler <rspangler@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>