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Kernel startup entry point requires disabling MMU and D-cache.

As for kexec-reboot, taking a close look at "msr sctlr_el1, x12" in
__cpu_soft_restart as the following:

-1. booted at EL1
The instruction is enough to disable MMU and I/D cache for
EL1 regime.

-2. booted at EL2, using VHE
Access to SCTLR_EL1 is redirected to SCTLR_EL2 in EL2.  So the instruction
is enough to disable MMU and clear I+C bits for EL2 regime.

-3. booted at EL2, not using VHE
The instruction itself can not affect EL2 regime. But The hyp-stub doesn't
enable the MMU and I/D cache for EL2 regime. And KVM also disable them for EL2
regime when its unloaded, or execute a HVC_SOFT_RESTART call. So when
kexec-reboot, the code in KVM has prepare the requirement.

As a conclusion, disabling MMU and clearing I+C bits in
SYM_CODE_START(arm64_relocate_new_kernel) is redundant, and can be removed
Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: James Morse <james.morse@arm.com>
Cc: Geoff Lev...