ramips: sunvalley - allow larger sysupgrade

ramips: sunvalley - allow larger sysupgrade
Aligned to size of mtd-concat partition (firmware)
  - in this device we have mtd-concat driver that joins multiple flash partitions
  - since sysupgrade works with mtd devices the rootfs partition is already joined
  - we can use a bigger sysupgrade image than factory/TFTP install images
Checked on hardware, no issues seen.
No modifications to images other than sysupgrade (i.e. TFTP / recovery images not touched).

Signed-off-by: Russell Morris <rmorris@rkmorris.us>