arm64: expose number of page table levels on Kconfig level

arm64: expose number of page table levels on Kconfig level

We would want to use number of page table level to define mm_struct.
Let's expose it as CONFIG_PGTABLE_LEVELS.

ARM64_PGTABLE_LEVELS is renamed to PGTABLE_LEVELS and defined before
sourcing init/Kconfig: arch/Kconfig will define default value and it's
sourced from init/Kconfig.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Acked-by: Catalin Marinas <[email protected]>
Cc: Will Deacon <[email protected]>
Tested-by: Guenter Roeck <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>