arm64: kexec: keep MMU enabled during kexec relocation

Now, that we have linear map page tables configured, keep MMU enabled
to allow faster relocation of segments to final destination.

Cavium ThunderX2:
Kernel Image size: 38M Iniramfs size: 46M Total relocation size: 84M
MMU-disabled:
relocation	7.489539915s
MMU-enabled:
relocation	0.03946095s

Broadcom Stingray:
The performance data: for a moderate size kernel + initramfs: 25M the
relocation was taking 0.382s, with enabled MMU it now takes
0.019s only or x20 improvement.

The time is proportional to the size of relocation, therefore if initramfs
is larger, 100M it could take over a second.

Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Tested-by: Pingfan Liu <piliu@redhat.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20210930143113.1502553-13-pasha.tatashin@soleen.com


Signed-off-by: Will Deacon <will@kernel.org>