microblaze: Fix sparse warnings - cache.c

Warning log:
CHECK   arch/microblaze/kernel/cpu/cache.c
arch/microblaze/kernel/cpu/cache.c:522:21: warning: symbol 'wb_msr' was not declared. Should it be static?
arch/microblaze/kernel/cpu/cache.c:538:21: warning: symbol 'wb_nomsr' was not declared. Should it be static?
arch/microblaze/kernel/cpu/cache.c:554:21: warning: symbol 'wt_msr' was not declared. Should it be static?
arch/microblaze/kernel/cpu/cache.c:569:21: warning: symbol 'wt_nomsr' was not declared. Should it be static?
arch/microblaze/kernel/cpu/cache.c:585:21: warning: symbol 'wt_msr_noirq' was not declared. Should it be static?
arch/microblaze/kernel/cpu/cache.c:600:21: warning: symbol 'wt_nomsr_noirq' was not declared. Should it be static?

Signed-off-by: Michal Simek <monstr@monstr.eu>