arm64/mm: Move PTE_VALID from SW defined to HW page table entry definitions

PTE_VALID signifies that the last level page table entry is valid and it is
MMU recognized while walking the page table. This is not a software defined
PTE bit and should not be listed like one. Just move it to appropriate
header file.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Steve Capper <steve.capper@arm.com>
Cc: Suzuki Poulose <suzuki.poulose@arm.com>
Cc: James Morse <james.morse@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>