devclass_add_driver: Permit NULL to be passed in dcp.

This permits a driver module structure that doesn't want to store a
pointer to the new driver's devclass.

Reviewed by: imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D34962

(cherry picked from commit 5bdea8826bdd48e2221f1b436ce86021ede412f2)