asm-generic offers an atomic-add based rwsem implementation, which
can avoid the need for heavier, spinlock-based synchronisation on the
fast path.

This patch makes use of the optimised implementation for arm64 CPUs.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>