rtc-cmos: look for PNP RTC first, then for platform RTC

We shouldn't rely on "pnp_platform_devices" to tell us whether there
is a PNP RTC device.

I introduced "pnp_platform_devices", but I think it was a mistake.
All it tells us is whether we found any PNPBIOS or PNPACPI devices.
Many machines have some PNP devices, but do not describe the RTC
via PNP.  On those machines, we need to do the platform driver probe
to find the RTC.

We should just register the PNP driver and see whether it claims anything.
If we don't find a PNP RTC, fall back to the platform driver probe.

This (in conjunction with the arch/x86/kernel/rtc.c patch to add
a platform RTC device when PNP doesn't have one) should resolve
these issues:

    http://bugzilla.kernel.org/show_bug.cgi?id=11580
    https://bugzilla.redhat.com/show_bug.cgi?id=451188

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Reported-by: Rik Theys <rik.theys@esat.kuleuven.be>
Reported-by:  <shr_msn@yahoo.com.tw>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>