Update mv_gpio driver to new FreeBSD API

[pdk_semihalf.com]

Summary

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.

[mw]

@manu I se you NACKed the patch, but don't see the actual remarks

[andrew]

In D14756#310616, @mw wrote:

@manu I se you NACKed the patch, but don't see the actual remarks

It's automatic by phabricator. @manu has it set up to add himself as a blocking reviewer on dts changes.

[mw]

@andrew Right, I looked more closely at the icon after posting my comment :)

[mw]

Hi @manu , do you have any objections to the patch?

[manu]

Hello Marcin, sorry, I've added a few comments.
I will look more later today.

sys/arm/mv/gpio.c

205

if (blah == 0)
return (ENXIO);

230

if (bootverbose)
device_printf(

332

mtx_destroy and bus_release resource is missing (maybe others)

[pdk_semihalf.com]

Fixed all issues pointed by manu

[manu]

Thanks.
You might want to add access_32/config_32 in a futur patch too.