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

Add the opcode for the H_WATCHDOG hypercall to hvcall.h.  While here,
add a definition for H_NOOP, a possible return code for H_WATCHDOG.
Signed-off-by: Scott Cheloha <cheloha@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220713202335.1217647-2-cheloha@linux.ibm.com