ARM: mm: don't use the access flag permissions mechanism for classic MMU

The simplified access permissions model is not used for the classic MMU
translation regime, so ensure that it is turned off in the sctlr prior
to turning on address translation for ARMv7.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>