powerpc/microwatt: Make microwatt_get_random_darn() static

Make microwatt_get_random_darn() static, because it can be.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211118004415.1706863-1-mpe@ellerman.id.au