arm64: Move TIF_* documentation to individual definitions

Some TIF_* flags are documented in the comment block at the top, some
next to their definitions, some in both places.

Move all documentation to the individual definitions for consistency,
and for easy lookup.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Will Deacon <will@kernel.org>