ARM: at91: debug: add lan966 support

Add support for low-level debugging on FLEXCOM USART of
LAN966 SoC.

Signed-off-by: Kavyasree Kotagiri <kavyasree.kotagiri@microchip.com>
Reviewed-by: Michael Walle <michael@walle.cc>
Tested-by: Michael Walle <michael@walle.cc>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220513092530.19213-1-kavyasree.kotagiri@microchip.com