watchdog: hpwdt (12/12): Make NMI decoding a compile-time option

hpwdt is quite functional without the NMI decoding feature.
This change lets users disable the NMI portion at compile-time
via the new HPWDT_NMI_DECODING config option.

Signed-off-by: dann frazier <dannf@hp.com>
Acked-by: Thomas Mingarelli <Thomas.Mingarelli@hp.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>