powerpc/mpc5121: Add generic board support for MPC5121 platforms

Move shared code from mpc5121_ads.c to new file mpc512x_shared.c
- mpc512x_find_ips_freq -> unchanged
- contents of mpc5121_ads_init_IRQ -> mpc512x_init_IRQ
  - looking for fsl,mpc5121-ipic instead of fsl,ipic
- mpc5121_ads_declare_of_platform_devices -> mpc5121_declare_of_platform_devices
  - and use compatible for lookup instead of node name

Add new generic board setup mpc5121_generic.c

Signed-off-by: John Rigby <jrigby@freescale.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>