mips: generic: add fdt fixup for Realtek reference board

The bootloader used on the Realtek RTL9302C boards is an ancient vendor
fork of U-Boot that doesn't understand device trees. So to run a modern
kernel it is necessary use one of the APPENDED_DTB options.

When appending the DTB the inintrd information, if present, needs to be
inserted into the /chosen device tree node. The bootloader provides the
initrd start/size via the firmware environment. Add a fdt fixup that
will update the device tree with the initrd information.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>