[MIPS] Make gcmp_probe() static

The gcmp_probe() function is needlessly defined global, and
this patch makes it static.

Tested by booting a Malta 4Kc board up to the shell prompt.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@movial.fi>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>