dt-bindings: serial: atmel,at91-usart: Add gclk as a possible USART clock

dt-bindings: serial: atmel,at91-usart: Add gclk as a possible USART clock

The Devicetree nodes for FLEXCOM's USART can also have an alternative
clock source for the baudrate generator (other than the peripheral
clock), namely the Generick Clock. Thus make the binding aware of
this clock that someone may place in the clock related
properties of the USART node.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220922113347.144383-6-sergiu.moga@microchip.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>