Suppress D_NEEDGIANT warnings for some drivers

During boot we warn that the kbd and openfirm drivers are Giant-locked
and may be deleted. Generally, the warning helps signal that certain
old drivers are not being maintained and are subject to removal, but
this doesn't really apply to certain drivers which are harder to
detangle from Giant.

Add a flag, D_GIANTOK, that devices can specify to suppress the
misleading warning. Use it in the kbd and openfirm drivers.

Reviewed by: imp, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30649