ARM: highbank: add missing get_tbclk

The get_tbclk function was missing and the recent commit "common: add
possibility for readline_into_buffer timeout" makes it required.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>

arch/arm/cpu/armv7/highbank/timer.c

@@ -121,3 +121,8 @@ ulong get_timer_masked(void)
 {
 	return tick_to_time(get_ticks());
 }
+
+ulong get_tbclk(void)
+{
+	return CONFIG_SYS_HZ;
+}