board: sl28: remove COUNTER_FREQUENCY_REAL

The frequency of the system counter is static which is given by the
COUNTER_FREQUENCY option. Remove COUNTER_FREQUENCY_REAL.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/include/configs/kontron_sl28.h b/include/configs/kontron_sl28.h
index 38063ba..df46e58 100644
--- a/include/configs/kontron_sl28.h
+++ b/include/configs/kontron_sl28.h
@@ -37,8 +37,6 @@
 /* serial port */
 #define CONFIG_SYS_NS16550_CLK          (get_bus_freq(0) / 2)
 
-#define COUNTER_FREQUENCY_REAL		(get_board_sys_clk() / 4)
-
 /* SPL */
 
 #define CONFIG_SYS_MONITOR_LEN		(1024 * 1024)