cleanup: introduce CONFIG_MPC830x

Introduce a new configuration token CONFIG_MPC830x to be shared among
mpc8308 and mpc8309. Define it for existing 8308 boards, and refactor
existing common code so to make future introduction of 8309 simpler.

Signed-off-by: Gerlando Falauto <gerlando.falauto@keymile.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
diff --git a/arch/powerpc/cpu/mpc83xx/speed.c b/arch/powerpc/cpu/mpc83xx/speed.c
index 3383568..4ad3ec3 100644
--- a/arch/powerpc/cpu/mpc83xx/speed.c
+++ b/arch/powerpc/cpu/mpc83xx/speed.c
@@ -185,7 +185,10 @@
 		/* unkown SCCR_TSEC1CM value */
 		return -2;
 	}
+#endif
 
+#if defined(CONFIG_MPC830x) || defined(CONFIG_MPC831x) || \
+	defined(CONFIG_MPC834x) || defined(CONFIG_MPC837x)
 	switch ((sccr & SCCR_USBDRCM) >> SCCR_USBDRCM_SHIFT) {
 	case 0:
 		usbdr_clk = 0;