sh: mach-sdk7786: Handle baseboard NMI source selection.

The on-board NMI switch is routed through and mangled by the FPGA prior
to its delivery to the NMI pin, so add some glue for the various
configuration options. The default is to unmask it and enable all input
sources.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>