cros_ec: Support systems with no EC interrupt

Some systems do not have an EC interrupt. Rather than assuming that the
interrupt is always present, and hanging forever waiting for more input,
handle the missing interrupt. This works by reading key scans only until
we get an identical one. This means the EC keyscan FIFO is empty.

Tested-by: Che-Liang Chiou <clchiou@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>