[MIPS] make plat_perf_setup() static

There is no need for the plat_perf_setup() function to be global,
so make it static.

Successfully build-tested using default configs for Malta, Atlas
and SEAD boards.

Runtime test successfully performed by booting the Malta 4Kc board
up to the shell prompt.
Signed-off-by: NDmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/mips-boards/generic/time.c

@@ -147,7 +147,7 @@ unsigned long read_persistent_clock(void)
 	return mc146818_get_cmos_time();
 }
 
-void __init plat_perf_setup(void)
+static void __init plat_perf_setup(void)
 {
 #ifdef MSC01E_INT_BASE
 	if (cpu_has_veic) {