powerpc: Use __REF macro instead of old .text.init.refok.

The section .text.init.refok is deprecated and __REF (.ref.text)
should be used in assembly files instead.  This patch cleans up a few
uses of .text.init.refok in the powerpc architecture.

Signed-off-by: Tim Abbott <tabbott@mit.edu>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>