clk: at91: sama5d2: Add Generic Clocks for UART/USART

Add the generic clocks for UART/USART in the sama5d2 driver to allow them
to be registered in the Common Clock Framework.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220913142205.162399-14-sergiu.moga@microchip.com