ARM: 6944/1: mm: allow ASID 0 to be allocated to tasks

Now that ASID 0 is no longer used as a reserved value, allow it to be
allocated to tasks.

Acked-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>