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: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>