Add support for "compatible" parameter in ofw_fdt_fixup

Sometimes it's convenient to provide fixup to many boards
that use the same SoC family (eg. Marvell Armada 38x).
Instead of putting multiple entries in fdt_fixup_table,
use one entry which refers to all boards with given SoC.

Submitted by: Patryk Duda <pdk@semihalf.com>
Reviewed by: nwhitehorn, cognet (mentor)
Approved by: cognet (mentor)
Obtained from: Semihalf
Differential Revision: https://reviews.freebsd.org/D11878