riscv32: sifive-freedom: Move dts.fixup into soc dir

The dts.fixup for the hifive1 board was not board specific but related
to the SoC.  Move the dts.fixup into the SoC dir so any other boards
using this same SoC will take advantage of it.

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