sifive_gpio: Add SiFive GPIO controller driver

This is present on both the FU540 and FU740, but only needed for the
FU740 in order to assert reset and power enable signals for its PCIe
controller.

Reviewed by: mhorne
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31031

(cherry picked from commit b47e5c5dbe2058b6a178230dc7796c37bfeaa926)