asm-generic: rwsem: de-PPCify rwsem.h

asm-generic/rwsem.h used to live under arch/powerpc. During its
liberation to common code, a few references to its former home where
preserved, in particular the definition of RWSEM_ACTIVE_MASK is
predicated on CONFIG_PPC64.

This patch updates the ifdefs and comments to architecturally neutral
versions.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Richard Kuo <rkuo@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>