dts: add support for soc family level fixups

We duplicate a lot of fixup info per board which could be done at the
SoC family level.  So introduce the concept of DTS_SOC_FIXUP_FILE which
we default to arch/<ARCH>/soc/<SOC_FAMILY>/<SOC_SERIES>/dts.fixup.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>