fdt: fix panic in fdt_slicer/geom due to recent geom changes

A recent change in GEOM ordering (c11b701915116bf512f3d77dfdac7a867f51ca0a -
"geom: Push GEOM sysinit ordering to after devctl" changed the GEOM init
ordering to SI_ORDER_THIRD. However, the FDT slicer / GEOM flash
slice code needs the GEOM subsystem to be initialised before it
starts loading in flash slices from FDT, and will panic because
the mutex isn't initialised.

So bump this now to SI_ORDER_FOURTH, so it occurs after the GEOM
initialisation.

Differential Revision: https://reviews.freebsd.org/D49692
Reviewed by: jhibbits