chromeec: Add google_chromeec_wait_for_display

The google_chromeec_wait_for_display API checks whether a display is
ready or not. It waits in a loop until EC says it entered DisplayPort
alternative mode or times out in 2 seconds.

BUG=b:72387533
BRANCH=none
TEST=See 23746 "mb/google/fizz: Wait until display is ready"

Change-Id: Ieee5db77bd6e147936ea8fc735dcbeffec98c0f8
Signed-off-by: Daisuke Nojiri <dnojiri@chromium.org>
Reviewed-on: https://review.coreboot.org/23745
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/ec/google/chromeec/ec.h

@@ -37,6 +37,7 @@ bool google_chromeec_is_uhepi_supported(void);
 int google_ec_running_ro(void);
 void google_chromeec_init(void);
 int google_chromeec_pd_get_amode(uint16_t svid);
+void google_chromeec_wait_for_display(void);
 
 /* Device events */
 uint64_t google_chromeec_get_device_enabled_events(void);