arm64: use correct register width when retrieving ASID

The ASID is represented as an unsigned int in mm_context_t and we
currently use the mmid assembler macro to access this element of the
struct. This should be accessed with a register of 32-bit width. If
the incorrect register width is used the ASID will be returned in
bits[32:63] of the register when running under big-endian.

Fix a use of the mmid macro in tlb.S to use a 32-bit access.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Matthew Leach <matthew.leach@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>