arm: mediatek: fix ram init for mt7622 (decrease to 1G)

RAM init on mt7622 based bananapi R64 is broken since v2023.10.

Increasing the mem-map does not help here, so i reduced the maximum
available ram in get_ram_size call from 2G to 1G (board has only 1G).

Fixes: 5fd6d4c7 ("arm: mediatek: retrieve ram_base from dts node for armv8 platform")
Signed-off-by: Frank Wunderlich <frank-w@public-files.de>