ARM: at91: debug: add sama7g5 low level debug uart

Add sama7g5 SoC debug uart on Flexcom3. This is the UART that the
ROM bootloader uses.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210409113116.482199-2-eugen.hristev@microchip.com