MIPS: allow override of get_tbclk()

Allow SoC or board layers with reconfigurable cpu clocks
capabilties to do implementation specific lookups and service
get_tbclk() requests.

Signed-off-by: Alex Nemirovsky <alex.nemirovsky@cortina-access.com>

arch/mips/cpu/time.c

@@ -13,7 +13,7 @@ unsigned long notrace timer_read_counter(void)
 	return read_c0_count();
 }
 
-ulong notrace get_tbclk(void)
+ulong notrace __weak get_tbclk(void)
 {
 	return CONFIG_SYS_MIPS_TIMER_FREQ;
 }