mpc5121: squash some fdt fixup errors

On ADS5121 when booting linux the following errors are seen:
    Unable to update property /soc5121@80000000:bus-frequency, err=FDT_ERR_NOTFOUND
    Unable to update property /soc5121@80000000/ethernet@2800:local-mac-address, err=FDT_ERR_NOTFOUND
    Unable to update property /soc5121@80000000/ethernet@2800:address, err=FDT_ERR_NOTFOUND

This is caused by ft_cpu_setup trying to deal with
both old and new soc node naming.  This patch
fixes this by being smarter about what to
fixup.

Also do soc node fixups by compatible instead of by path.
A new board config called OF_SOC_COMPAT defined
to be "fsl,mpc5121-immr" replaces the old
OF_SOC node path that was defined to be "soc@80000000".

Old device trees still work, but the compatiblity
is conditional on CONFIG_OF_SUPPORT_OLD_DEVICE_TREES
which is on by default in include/configs/ads5121.h.

Signed-off-by: John Rigby <jrigby@freescale.com>