bhnd(4): Add a basic ChipCommon GPIO driver sufficient to support bwn(4)

The driver is functional on both BHND Wi-Fi adapters and MIPS SoCs, but
does not currently include support for features not required by bwn(4),
including GPIO interrupt handling.

Approved by: adrian (mentor, implicit)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12708