dt-bindings: mfd: cros-ec: Add compatible string for UART support

Add a compatible string to support the UART implementation of the cros
ec interface. The driver does not support the reg and interrupt
properties, so exempt them from being required for UART compatible nodes.

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>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20221207104005.v10.2.I9e018ecb8bdf341648cb64417085978ff0d22a46@changeid