This is mostly a port of dbf62d50 ("ARM: mm: introduce L_PTE_VALID
for page table entries") and 26ffd0d4 ("ARM: mm: introduce present,
faulting entries for PAGE_NONE") from ARM, which makes use of present,
faulting page table entries for page table entries mapped as PROT_NONE.

The main difference with this implementation is that we can make use of
the two pte type bits in order to avoid allocating a software bit for
identifying PROT_NONE pages, instead reserving the 10b suffix for these
types of mappings.

This is required to prevent users from accessing such pages via syscalls
such as read/write over a pipe.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>