arm64: Use INIT_SCTLR_EL1_MMU_OFF to disable the MMU on CPU restart

Instead of doing a RMW on SCTLR_EL1 to disable the MMU, use the
existing define that loads the right set of bits.

Acked-by: Will Deacon <will@kernel.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>