leds: qpnp-wled: Add sysfs property to enable/disable the irq

WLED interrupts need to be disabled and enabled during the entry and
exit of the secure display use case respectively. Add sysfs property
to allow the userspace to perform this operation.

To disable the interrupts:
 echo 1 > /sys/class/leds/wled/secure_mode

To enable the interrupts:
 echo 0 > /sys/class/leds/wled/secure_mode

Change-Id: Iedf70bcf2fecefcd2e58c213a17bd8b545f93c7a
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>