sh: Preliminary SDK7786 board support.

This stubs in some preliminary board support for the RTE SDK7786.

This is quite stunted at the moment, and primarily builds on top of the
system FPGA. FPGA IRQs are handled via CPU IRL masking for simplicity,
with initial peripheral support restricted to the debug ethernet.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>