ipq806x: Enlarge R7500v2 flash - use netgear partition

ipq806x: Enlarge R7500v2 flash - use netgear partition

This patch increases available space from 20MB to 86MB.
Netgrear OEM firmware contains a 'netgear' partition followed 'ubi',
which can be used in openwrt for larger ubi space. (similar to
Netgear R7800).

original author (notmyrealhandle) tested this on R7500v2 and point that
OEM firmware can auto rebuild this partition (if used by openwrt).

Author: notmyrealhandle<22336358+notmyrealhandle@users.noreply.github.com>
Signed-off-by: Wang Xin-yu (王昕宇) <comicfans44@gmail.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com> [minor touch-ups]