fdt: refactor initrd related code

Created a new fdt_initrd() to deal with setting the initrd properties
in the device tree and fixing up the mem reserve.  We can use this
both in the choosen node handling and lets us remove some duplicated
code when we fixup the initrd info in bootm on PPC.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>