microblaze: Fix sparse warning - timer.c

Variables and init_microblaze_timecounter should be static.

Warning log:
CHECK   arch/microblaze/kernel/timer.c
arch/microblaze/kernel/timer.c:41:14: warning: symbol 'freq_div_hz' was not declared. Should it be static?
arch/microblaze/kernel/timer.c:42:14: warning: symbol 'timer_clock_freq' was not declared. Should it be static?
arch/microblaze/kernel/timer.c:205:12: warning: symbol 'init_microblaze_timecounter' was not declared. Should it be static?

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