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

s,KOBJMETHOD_END,DEVMETHOD_END,g in order to fully hide the explicit mention
of kobj(9) from device drivers.

Style changes

Obtained from: NetBSD (mc146818reg.h)

Don't probe the bq4802 variant found in Ultra 25 and 45 for now as
this chip isn't MC146818 compatible and requires different handlers
(but which I can't test due to lack of such hardware).

Take advantage of KOBJMETHOD_END.

If clock_ct_to_ts fails to convert time time from the real time clock,
print a one line error message. Add some comments on not being able to
trust the day of week field (I'll act on these comments in a follow up
commit).

Approved by: re
MFC after: 3 weeks

- Flesh out the support for the EBus variant which actually is the
RTC function of a National Semiconductor PC87317/PC97317. This
consists of using the century register the same way Solaris does
for compatibility reasons. Once there is a MD power(4) we'd also
want to interface the APC (Advanced Power Control) functionality
of the same chip function with it.
- Use a macro for the device description and take advantage of
ISA_PNP_PROBE() setting the device description.
- Use the generated typedefs for the prototypes of the device
interface functions.

Remove superfluous bzero()'ing of the softc.

After some input from bde@ and rereading the datasheet use a MTX_SPIN
mutex instead of a MTX_DEF one in order to defer preemption while
reading the date and time registers. If we don't manage to read them
within the time slot where we are guaranteed that no updates occur we
might actually read them during an update in which case the output is
undefined.

o mc146818(4):
- Add locking.
- Account for if the MC146818_NO_CENT_ADJUST flag is set we don't need
to check wheter year < POSIX_BASE_YEAR.
- Add some comments about mapping the day of week from the range the
generic clock code uses to the range the chip uses and which I meant
to add in the initial version.
- Minor clean-up, use __func__ instead of hardcoded function names in
error strings.

o in the rtc(4) front-end additionally:
- Don't leak resources in case mc146818_attach() fails.
- Account for ebus(4) defaulting to SYS_RES_MEMORY for the memory
resources since ebus.c rev. 1.22.

/* -> /*- for license, minor formatting changes

Add a front-end for the `rtc' device which is a MC146818 compatible
clock found on the ISA bus (some USIIe, USIIi and USIIIi models) and
EBus (USIII models) instead of a MK48Txx clock.

Testet by: Matthew T. Lager" <freebsd@trinetworks.com> on Sun Fire V100,
Xavier Beaudouin <kiwi@oav.net> on Netra X1 (initial version)