sh: mach-sdk7786: Probe system FPGA area mapping.

This implements dynamic probing for the system FPGA. The system reset
controller contains a fixed magic read word in order to identify the
FPGA. This just utilizes a simple loop that scans across all of the fixed
physical areas (area 0 through area 6) to locate the FPGA.

The FPGA also contains register information detailing the area mappings
and chip select settings for all of the other blocks, so this needs to be
done before we can set up anything else.

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