arm64: asm/pgtable.h pmd/pud definitions clean-up

Non-functional change to group together the pmd/pud definitions and
reduce the amount of #if CONFIG_ARM64_PGTABLE_LEVELS.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Jungseok Lee <jungseoklee85@gmail.com>