KVM: arm64: Turn SCTLR_ELx_FLAGS into INIT_SCTLR_EL2_MMU_ON

Only the nVHE EL2 code is using this define, so let's make it
plain that it is EL2 only, and refactor it to contain all the
bits we need when configuring the EL2 MMU, and only those.

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