arm64: mm: move fixmap code to its own file

Over time, arm64's mm/mmu.c has become increasingly large and painful to
navigate. Move the fixmap code to its own file where it can be understood in
isolation.

There should be no functional change as a result of this patch.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20230406152759.4164229-3-mark.rutland@arm.com


Signed-off-by: Will Deacon <will@kernel.org>