watchdog: Move watchdog from being x86 specific to common code

watchdog: Move watchdog from being x86 specific to common code

Augment watchdog_setup() to be able to possibly return an error, and introduce
watchdog_enabled() as a better alternative to knowing the architectures
internal details.

This patch does not change the x86 implementaion, beyond making it compile.

For header files, some includes of xen/nmi.h were only for the watchdog
functions, so are replaced rather than adding an extra include of
xen/watchdog.h

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>