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 watchdogs. 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

Orabug: 24796651

Signed-off-by: Babu Moger <babu.moger@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 43e96774e0a338e883e9ced9e717424df126b153)
Signed-off-by: Allen Pais <allen.pais@oracle.com>