mpc83xx: Replace CONFIG_83XX_CLKIN in calculations

CONFIG_SYS_CLK_FREQ is the standard way to set the system clock
frequency. On MPC83xx, CONFIG_83XX_CLKIN is used for this purpose.
Hence, the obvious way is to replace CONFIG_83XX_CLKIN with
CONFIG_SYS_CLK_FREQ.

A few MPC83xx boards use the CONFIG_83XX_CLKIN variable for computing
CONFIG_SYS_NS16550_CLK. This makes it harder to replace
CONFIG_83XX_CLKIN.

But the value of the multiplicator can be read from the SPMR register.

Hence, replace the static calculations with a call to a new get_bus_freq
function, as other architectures do.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
diff --git a/include/configs/MPC8315ERDB.h b/include/configs/MPC8315ERDB.h
index c373a0a..adb2bb6 100644
--- a/include/configs/MPC8315ERDB.h
+++ b/include/configs/MPC8315ERDB.h
@@ -271,7 +271,7 @@
  */
 #define CONFIG_SYS_NS16550_SERIAL
 #define CONFIG_SYS_NS16550_REG_SIZE	1
-#define CONFIG_SYS_NS16550_CLK		(CONFIG_83XX_CLKIN * 2)
+#define CONFIG_SYS_NS16550_CLK		(get_bus_freq(0))
 
 #define CONFIG_SYS_BAUDRATE_TABLE  \
 		{300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200}