kernel: mtdsplit_uimage: replace "netgear, uimage" parser

kernel: mtdsplit_uimage: replace "netgear, uimage" parser

The "netgear,uimage" parser can be replaced by the generic
parser using device specific openwrt,ih-magic and
openwrt,ih-type properties.

Device tree properties for the following devices have not
been set, as they have been dropped from OpenWrt with the
removal of the ar71xx target:

 FW_MAGIC_WNR2000V1     0x32303031
 FW_MAGIC_WNR2000V4     0x32303034
 FW_MAGIC_WNR1000V2_VC  0x31303030
 FW_MAGIC_WPN824N       0x31313030

Tested-by: Sander Vanheule <sander@svanheule.net>  # WNDR3700v2
Tested-by: Stijn Segers <foss@volatilesystems.org> # WNDR3700v1
Signed-off-by: Bjørn Mork <bjorn@mork.no>