cpu: 83xx: Add missing dependency on CPU_MPC83XX

It looks quite weird that for non PPC platforms cpu driver for MPC83xx can
be selected. That's why define proper dependency.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
diff --git a/drivers/cpu/Kconfig b/drivers/cpu/Kconfig
index 3d5729f..7897281 100644
--- a/drivers/cpu/Kconfig
+++ b/drivers/cpu/Kconfig
@@ -9,7 +9,7 @@
 
 config CPU_MPC83XX
 	bool "Enable MPC83xx CPU driver"
-	depends on CPU
+	depends on CPU && MPC83xx
 	select CLK_MPC83XX
 	help
 	  Support CPU cores for SoCs of the MPC83xx series.