Add sun4v_wdt watchdog driver

Add sun4v_wdt watchdog driver

This driver adds sparc hypervisor watchdog support. Timeout is set in
milliseconds since that is the granularity supported and it honors
the settings of both the watchdog-resolution and watchdog-max-timeout
MD properties.

Note that most watchdog drivers use timeout in seconds. This driver
requires timeout_ms as a module parameter and time in ms.

In this driver, the default is 60000ms or 60 seconds.

This driver also modifies hvcalls.S and changes sun4v_mach0_set_watchdog
such that it allows for NULL to be passed as 2nd parameter. This removes
the need to pass &time_remaining which is not useful.

Signed-off-by: Wim Coekaerts <wim.coekaerts@oracle.com>