ARM: mach-at91: update alternate function of signal PD20

The alternate function of PD20 is 4 as per the datasheet of
sama7g5 and not 5 as defined earlier.

Fixes: 558378a4 ("ARM: mach-at91: add support for new SoC sama7g5")
Signed-off-by: Hari Prasath <Hari.PrasathGE@microchip.com>
Reviewed-by: Eugen Hristev <eugen.hristev@microchip.com>