ARM: hotplug cpu: ensure that __enable_mmu is identity mapped

__enable_mmu is required to be executed in an identity mapped region
to ensure that variances in CPUs do not cause a crash.  We currently
achieve this by assuming that it will be co-located with
__create_page_tables.  With hotplug CPU support, this assumption
becomes invalid.  Implement a better solution which ensures that
it will be appropriately mapped no matter where it is placed.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>