ARM: 7926/1: mm: flesh out and fix the comments in the ASID allocator

The ASID allocator has to deal with some pretty horrible behaviours by
the CPU, so expand on some of the comments in there so I remember why
we can never allocate ASID zero to a userspace task.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>