On AArch64 the TCR_EL1.TBI0 bit is set by default, allowing userspace
(EL0) to perform memory accesses through 64-bit pointers with a non-zero
top byte. Introduce the document describing the relaxation of the
syscall ABI that allows userspace to pass certain tagged pointers to
kernel syscalls.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Szabolcs Nagy <szabolcs.nagy@arm.com>
Acked-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
Co-developed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>