[chvgpio] add GPIO driver for Intel Z8xxx SoC family

Add chvgpio(4) driver for Intel Z8xxx SoC family. This product
was formerly known as Cherry Trail but Linux and OpenBSD drivers
refer to it as Cherry View. This driver is derived from OpenBSD
one so the name is kept for alignment with another BSD system.

Submitted by: Tom Jones <tj@enoti.me>
Reviewed by: gonzo, wblock(man page)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D13086