powerpc: mpc83xx: Implement get_serial_clock()

DM serial drivers on PowerPC determine their clock frequency via the
get_serial_clock function. This function is not Implemented yet for
MPC83xx.

This patch Implements the function so that DM serial drivers work on
MPC83xx.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/arch/powerpc/cpu/mpc83xx/speed.c b/arch/powerpc/cpu/mpc83xx/speed.c
index e870a23..e118a10 100644
--- a/arch/powerpc/cpu/mpc83xx/speed.c
+++ b/arch/powerpc/cpu/mpc83xx/speed.c
@@ -516,6 +516,11 @@
 	return gd->mem_clk;
 }
 
+int get_serial_clock(void)
+{
+	return get_bus_freq(0);
+}
+
 static int do_clocks(cmd_tbl_t *cmdtp, int flag, int argc, char * const argv[])
 {
 	char buf[32];