mpc85xx: tl-wdr4900-v1: convert to mtdsplit image

Currently, the image creation process for the TP-Link tl-wdr4900-v1
needs a fixed sized kernel and places the rootfs partition at a
fixed offset. With the upcoming move to 4.19 the kernel will no
longer fit into the existing allocated space for the kernel
partition.

This patch converts the device to utilize the established
tplink,firmware mtdsplitter, which can deal with a dynamic
kernel/rootfs size.

Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [reworded commit]