[PATCH] Clean up usage of boot_dev

dev_t boot_dev is declared in arch/powerpc/kernel/setup_32.c
and in arch/powerpc/kernel/setup_64.c but not used in these files.
It is only used in arch/powerpc/platforms/powermac/setup.c, so make
it static in this file.

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>