Add support for an array of hwresets

In some drivers we need to assert and deassert a group of hardware
resets in any order. To support this add a new hwreset_array that
manages all hwresets defined for a device.

Reviewed by: bz, manu, mmel
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37357

(cherry picked from commit 969935b86b179b2b517ab5d35d943fcb761203c1)