arm: cstart64.S: Remove icache invalidation from asm_mmu_enable

According to the ARM ARM [1]:

"In Armv8, any permitted instruction cache implementation can be
described as implementing the IVIPT Extension to the Arm architecture.

The formal definition of the Arm IVIPT Extension is that it reduces the
instruction cache maintenance requirement to the following condition:
Instruction cache maintenance is required only after writing new data to
a PA that holds an instruction".

We never patch instructions in the boot path, so remove the icache
invalidation from asm_mmu_enable. Tests that modify instructions (like
the cache test) should have their own icache maintenance operations.

[1] ARM DDI 0487E.a, section D5.11.2 "Instruction caches"

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>