serial: samsung: dt-bindings: Add reg-io-width compatible

Add a description for reg-io-width options for the samsung serial
UART peripheral.

Signed-off-by: Hyunki Koo <hyunki00.koo@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200506080242.18623-2-hyunki00.koo@samsung.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>