watchdog/pseries-wdt: initial support for H_WATCHDOG-based watchdog timers

PAPR v2.12 defines a new hypercall, H_WATCHDOG. The hypercall permits
guest control of one or more virtual watchdog timers. The timers have
millisecond granularity. The guest is terminated when a timer
expires.

This patch adds a watchdog driver for these timers, "pseries-wdt".

pseries_wdt_probe() currently assumes the existence of only one
platform device and always assigns it watchdogNumber 1. If we ever
expose more than one timer to userspace we will need to devise a way
to assign a distinct watchdogNumber to each platform device at device
registration time.

Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220713202335.1217647-5-cheloha@linux.ibm.com