regulator: qpnp-oledb: Add sysfs class to enable/disable secure mode

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

To enter the secure mode:
	echo 1 > /sys/class/lcd_bias/secure_mode

To exit the secure mode:
	echo 0 > /sys/class/lcd_bias/secure_mode

Change-Id: If1e5d85961df4b25a4596c2b291c990f3e7c3a9c
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>