kernel: drop mips highmem offset start overrides

kernel: drop mips highmem offset start overrides

The maximum offset that can be supported is 0x20000000
Do not override it to to something bigger than that on MT7621, as that could
cause issues based on the fixed memory mappings. This makes the last 64 MB
RAM unusable on MT7621 devices with 512 MB but avoids incurring a heavy
performance hit

Fixes: cd2b74e01e8d ("ramips: mt7621: disable highmem support and remove highmem offset patch")
Signed-off-by: Felix Fietkau <nbd@nbd.name>
(cherry picked from commit a110de8152df46a1e2adf7010ba75fb3b1236cd8)