powerpc/512x: Group mpc512x board's selection menu

Allow board selection in a drop-down board sub-menu
like many other platforms do.

Before the patch:
...
[ ] Freescale MPC5121E ADS
[ ] Generic support for simple MPC5121 based boards
[ ] 52xx-based boards
...

Patched:
...
[*] 512x-based boards
[ ]   Freescale MPC5121E ADS
[ ]   Generic support for simple MPC5121 based boards
[ ] 52xx-based boards
...

This is a cleanup before adding new board selection entry.
Signed-off-by: NAnatolij Gustschin <agust@denx.de>
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

arch/powerpc/platforms/512x/Kconfig

 config PPC_MPC512x
-	bool
+	bool "512x-based boards"
+	depends on 6xx
 	select FSL_SOC
 	select IPIC
 	select PPC_CLOCK
 	select PPC_PCI_CHOICE
 	select FSL_PCI if PCI
 
-config PPC_MPC5121
-	bool
-	select PPC_MPC512x
-
 config MPC5121_ADS
 	bool "Freescale MPC5121E ADS"
-	depends on 6xx
+	depends on PPC_MPC512x
 	select DEFAULT_UIMAGE
-	select PPC_MPC5121
 	select MPC5121_ADS_CPLD
 	help
 	  This option enables support for the MPC5121E ADS board.
 
 config MPC5121_GENERIC
 	bool "Generic support for simple MPC5121 based boards"
-	depends on 6xx
+	depends on PPC_MPC512x
 	select DEFAULT_UIMAGE
-	select PPC_MPC5121
 	help
 	  This option enables support for simple MPC5121 based boards
 	  which do not need custom platform specific setup.