sparc: implement watchdog_nmi_enable and watchdog_nmi_disable

sparc: implement watchdog_nmi_enable and watchdog_nmi_disable

Implement functions watchdog_nmi_enable and watchdog_nmi_disable to
enable/disable nmi watchdog.  Sparc uses arch specific nmi watchdog
handler.  Currently, we do not have a way to enable/disable nmi watchdog
dynamically.  With these patches we can enable or disable arch specific
nmi watchdogs using proc or sysctl interface.

Example commands.
To enable: echo 1 >  /proc/sys/kernel/nmi_watchdog
To disable: echo 0 >  /proc/sys/kernel/nmi_watchdog

It can also achieved using the sysctl parameter kernel.nmi_watchdog

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Babu Moger <[email protected]>
Acked-by: Don Zickus <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Kosina <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Yaowei Bai <[email protected]>
Cc: Aaron Tomlin <[email protected]>
Cc: Ulrich Obergfell <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Hidehiro Kawai <[email protected]>
Cc: Josh Hunt <[email protected]>
Cc: "David S. Miller" <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>