powerpc/512x: move mpc5121_generic platform to mpc512x_generic.

This provides a base for using 512x_generic platform on mpc5125 boards.

By this way 512x_GENERIC it could be used for all generic mpc512x boards
and kernel could be compiled with mpc512x_defconfig.

Signed-off-by: Matteo Facchinetti <matteo.facchinetti@sirius-es.it>
[agust: applied s/mpc5121/mpc512x in mpc512x_generic.c]
Signed-off-by: Anatolij Gustschin <agust@denx.de>