ramips: add missing information to dlink headers

Add additional header information required for newer
bootloaders found on DIR-2660-A1 & A2.

Also remove the MTD splitter compatible from the second firmware
partition, as OpenWrt only supports handling of the first one.

Signed-off-by: Alan Luck <luckyhome2008@gmail.com>
[rephrase commit message, remove removal of read-only flags]
Signed-off-by: David Bauer <mail@david-bauer.net>