sh: mach-sdk7786: FPGA updates.

This does a bit of refactoring of the FPGA management code. The primary
FPGA initialization is moved out to its own file in preparation for
implementing some of the more complex capabilities, a complete set of
register definitions is provided, and all of the existing users in the
board code are moved over to use the new interface instead of setting up
overlapping mappings. This also corrects the FPGA size, which previously
was chomped off at the SDIF control register.

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