MFC r349883: owc_gpiobus: small formatting cleanup

MFC r349428,r349433: owc_gpiobus: clean / fix up the driver module things

"fdt" is removed from the driver module name as the driver does not
require FDT and can work very well on hints based systems.

A module dependency is added for gpiobus. Without that owc cannot
resolve symbols in gpiobus if both are loaded as kernel modules.

Finally, a driver module module version is added.

MFC r321686:

Add inline functions to convert between sbintime_t and decimal time units.
Use them in some existing code that is vulnerable to roundoff errors.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

New 1-Wire bus implementation. 1-Wire controller is abstracted, though
only gpiobus configured via FDT is supported. Bus enumeration is
supported. Devices are created for each device found. 1-Wire
temperature controllers are supported, but other drivers could be
written. Temperatures are polled and reported via a sysctl. Errors
are reported via sysctl counters. Mis-wired bus detection is included
for more trouble shooting. See ow(4), owc(4) and ow_temp(4) for
details of what's supported and known issues.

This has been tested on Raspberry Pi-B, Pi2 and Beagle Bone Black
with up to 7 devices.

Differential Revision: https://reviews.freebsd.org/D2956
Relnotes: yes
MFC after: 2 weeks
Reviewed by: loos@ (with many insightful comments)