powerpc/booke: move fdt loading to rom infrastructure

It's convinent to use rom to checking overlap, to reset etc.
And uImage and ramdisk loading has already moved to it.

Also, after we add fdt to rom, free it.

Signed-off-by: Liu Yu <yu.liu@freescale.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
