bump FreeBSD_version to 1500060 for gpiobus changes

[vexeduxr]

Summary

Bump FreeBSD_version and add UPDATING entry for gpiobus changes.

[imp]

So you don't need to get this reviewed (though it's good. you are doing it).

But The commit message should list the new version number. Since it's only to sys/param.h, it should be in the title:

param.h: Bump FreeBSD_version to 15000XX for gpiobus changes

and likely the rest of the commit message is fine.

[vexeduxr]

Update and rebase.

[vexeduxr]

In D51579#1181466, @imp wrote:

param.h: Bump FreeBSD_version to 15000XX for gpiobus changes

Done. Though I dropped the "param.h" prefix since there's an UPDATING entry now.

[imp]

Changes look good.