microblaze: Add static qualifiers

Adding static to internal variables and functions.

Sparse warnings:
arch/microblaze/kernel/prom.c:55:6: warning:
	symbol 'stdout' was not declared. Should it be static?
arch/microblaze/kernel/prom.c:57:12: warning:
	symbol 'early_init_dt_scan_chosen_serial' was not declared. Should it be static?
  CC      arch/microblaze/kernel/prom.o
arch/microblaze/kernel/intc.c:102:5: warning:
	symbol 'xintc_map' was not declared. Should it be static?
  CC      arch/microblaze/kernel/intc.o

Signed-off-by: Michal Simek <michal.simek@xilinx.com>