Do not require strict compatibility on simplebus

Strict compatibility requirement is a root of problems when simplebus'
node has two compatibility strings (i.e. on Armada38x). Removing this
requirement should not interfere with other platforms.

fdt_is_compatible_strict() and fdt_find_compatible() calls were changed
in fdt_common.c and mv_common.c.

Reviewed by: ian, imp
Obtained from: Semihalf
Sponsored by: Stormshield
Submitted by: Bartosz Szczepanek <bsz@semihalf.com>
Differential revision: https://reviews.freebsd.org/D4602