ramips: mt7621: disable highmem support and remove highmem offset patch

ramips: mt7621: disable highmem support and remove highmem offset patch

On MT7621 it was observed, that enabling highmem support causes a significant
performance drop, as documented in: https://github.com/openwrt/openwrt/issues/13151
By adjusting the highmem start offset, we avoid leaving any RAM unaddressable,
even on devices with 512 MB

Fixes: https://github.com/openwrt/openwrt/issues/13151
Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit cd2b74e01e8d5f5b80b82db1cb204c13ed99dd58)