ipq806x: increase kernel partition size for the TP-Link Archer VR2600v

The default image does not fit 2 MB anymore, expand kernel partition
to 3 MB.

Upgrading should work transparently via sysupgrade in both directions.
Another option would be to merge "kernel" and "rootfs" into a single
"firmware" partition using MTD_SPLIT_TPLINK_FW, but just changing the
sizes of the existing partitioning has been deemed safer in the absence
of an actual runtime test on an affected device; the maximum for rootfs
changes from 10.4 MB to 9.4 MB.

This change follows the example for the TP-Link Archer C2600, but has
not been runtime tested on a TP-Link Archer VR2600v.

Signed-off-by: Stefan Lippers-Hollmann <s.l-h@gmx.de>
Cc: Sebastian Quilitz <zeraphim@x-pantion.de>