arm64: Use macros instead of hard-coded constants for MAIR_EL1

Currently, the arm64 __cpu_setup has hard-coded constants for the memory
attributes that go into the MAIR_EL1 register. Define proper macros in
asm/sysreg.h and make use of them in proc.S.

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