arm64/mm: Use ESR_ELx_FSC macro while decoding fault exception

Just replace hard code value of 63 (0x111111) with an existing macro
ESR_ELx_FSC when parsing for the status code during fault exception.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>