powerpc: fix implicit use of cache.h in kernel/firmware.c

This file only needs export.h to get EXPORT_SYMBOL, but in doing
so, it uncovers an implicit use of linux/cache.h as follows:

 CC      arch/powerpc/kernel/firmware.o
arch/powerpc/kernel/firmware.c:20: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__read_mostly'
arch/powerpc/kernel/firmware.c:21: error: expected '=', ',', ';', 'asm' or '__attribute__' before '__used'
make[2]: *** [arch/powerpc/kernel/firmware.o] Error 1

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>