powerpc/pseries: register pseries-wdt device with platform bus

PAPR v2.12 defines a new hypercall, H_WATCHDOG.  The hypercall permits
guest control of one or more virtual watchdog timers.

These timers do not conform to PowerPC device conventions.  They are
not affixed to any extant bus, nor do they have full representation in
the device tree.

As a workaround we represent them as platform devices.

This patch registers a single platform device, "pseries-wdt", with the
platform bus if the FW_FEATURE_WATCHDOG flag is set.

A driver for this device, "pseries-wdt", will be introduced in a
subsequent patch.

Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220713202335.1217647-4-cheloha@linux.ibm.com