[PATCH] Remove occurences of PPC_MULTIPLATFORM in head_64.S

Since iSeries is merged to MULTIPLATFORM, there is no way to build a 64bit
kernel without MULTIPLATFORM, so PPC_MULTIPLATFORM can be removed in
64bit-only files.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>