arm64/hugetlb: Use macros for contiguous huge page sizes

Replace all open encoded contiguous huge page size computations with
available macro encodings CONT_PTE_SIZE and CONT_PMD_SIZE. There are other
instances where these macros are used in the file and this change makes it
consistently use the same mnemonic.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>