of: document /sys/firmware/fdt

Add ABI documentation for /sys/firmware/fdt

Update contact email for /sys/firmware/devicetree/* and add mail list

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Acked-by: Grant Likely <grant.likely@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>