microblaze: Use static inline functions in cacheflush.h

Using static inline functions ensure proper type checking
which also remove compilation warning for no MMU

Compilation warning:
arch/microblaze/include/asm/cacheflush.h: warning: 'addr'
 may be used uninitialized in this function [-Wmaybe-uninitialized]

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