x86: register a platform RTC device if PNP doesn't describe it

Most if not all x86 platforms have an RTC device, but sometimes the RTC
is not exposed as a PNP0b00/PNP0b01/PNP0b02 device in PNPBIOS or ACPI:

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

It's best if we can discover the RTC via PNP because then we know
which flavor of device it is, where it lives, and which IRQ it uses.

But if we can't, we should register a platform device using the
compiled-in RTC_PORT/RTC_IRQ resource assumptions.

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>