MIPS: bootm: rework and fix broken bootm code

The move to 'generic board' as well as changes in the generic
bootm code broke the boot of FIT uImage's. Especially uImage's
with additional initramfs images or FDT's do not work anymore.

Refactor the bootm code to work again with the generic bootm code.

Always relocate ramdisk and FDT in step 'bootm prep' because the
generic bootm code does this only for legacy uImage's.

Move the step 'bootm cmdline' to 'bootm prep' because the Linux
kernel parameters rd_start and rd_size have to be initialized after
the relocation of the ramdisk.

Furthermore support the step 'bootm fake'.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>