ar71xx: update Carambola2 platform data

Change list:
 * Remove button info on GPIO12, there is no button there.
 * Remove nvram mtd partition, as it's not used for anything, saves 64k for user data.

Tested building for carambola2 target.

Signed-off-by: Mantas Pucka <mantas@8devices.com>

Backport of r41993

git-svn-id: svn://svn.openwrt.org/openwrt/branches/barrier_breaker@42013 3c298f89-4303-0410-b956-a3cf2f4a3e73