ARM: rwsem: use asm-generic rwsem implementation

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 ARM CPUs.

Conflicts:
	arch/arm/Kconfig

Change-Id: I7db2705d8eca6a3f8c97e87a5e4632a885cf5882
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>