firmware: microchip: support writing bitstream info to flash

Updating the FPGA image might bring with it changes visible to Linux,
so it is helpful to also co-locate dt-overlays that describe the new
image contents. If these are packaged in a specific format [1] (detected
by first 4 bytes being MCHP, since FPGA images have no magic), load
the file to the reserved 1 MiB region immediately after the directory in
flash. The Beagle-V Fire's "gateware" already creates these files and
puts them in flash [2].

Link: https://github.com/polarfire-soc/polarfire-soc-documentation/blob/master/how-to/re-programming-the-fpga-from-linux.md#main-header-format
Link: https://openbeagle.org/beaglev-fire/gateware/-/blob/main/gateware_scripts/generate_gateware_overlays.py?ref_type=heads

 [2]
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>