Update mv_gpio driver to new FreeBSD API

This patch implements and exports functions described
in gpio_if.m file. It also uses new gpiobus_attach_bus function
instead of adding gpioc and gpiobus as children. It removes
ulgy reading SoC ID and related if..else, so it depends only on
data read from FDT.

Submitted by: Patryk Duda <pdk@semihalf.com>
Reviewed by: manu
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D14756