ARM: dts: at91: sam9x60: Specify the FIFO size for the Flexcom UART

The UART submodule in Flexcom has 16-byte Transmit and Receive FIFOs.

Signed-off-by: Manikandan Muralidharan <manikandan.m@microchip.com>
Signed-off-by: Durai Manickam KR <durai.manickamkr@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/20230207110651.197268-5-durai.manickamkr@microchip.com