Add dev.fb.X.resync sysctl to resync ARM framebuffer with VideoCore

Some applications (e.g. Kodi) use tvservice APIs to manage HDMI
modes, power state, EDID etc. directly through VideoCore. After
these manipulations VideoCore may loose its state and needs to be
resynced with ARM. Under Linux this problem is worked around using
fbset utility that recreates framebuffer. Since there is no fbset
utility in FreeBSD we provide sysctl for userland apps to get system
back into normal mode.