platform/chrome: cros_ec_uart: Add DT enumeration support

Existing firmware uses the "PRP0001" _HID and an associated compatible
string to enumerate the cros_ec_uart.

Add DT enumeration support for already shipped firmware.

Signed-off-by: Bhanu Prakash Maiya <bhanumaiya@chromium.org>
Co-developed-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Mark Hasemeyer <markhas@chromium.org>
Reviewed-by: Prashant Malani <pmalani@chromium.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/20221227123212.v13.3.Ie23c217d69ff25d7354db942613f143bbc8ef891@changeid