mips: time: Only compile the weak get_tbclk() when needed

This patch opts-out the compilation of get_tbclk() if
CONFIG_SYS_MIPS_TIMER_FREQ is not defined. This is used on the Octeon
platform, where the weak get_tbclk() function is overwritten by its
platform specific one.

Signed-off-by: Stefan Roese <sr@denx.de>
diff --git a/arch/mips/cpu/time.c b/arch/mips/cpu/time.c
index e0c1868..5e7a714 100644
--- a/arch/mips/cpu/time.c
+++ b/arch/mips/cpu/time.c
@@ -13,7 +13,9 @@
 	return read_c0_count();
 }
 
+#if defined(CONFIG_SYS_MIPS_TIMER_FREQ)
 ulong notrace __weak get_tbclk(void)
 {
 	return CONFIG_SYS_MIPS_TIMER_FREQ;
 }
+#endif