sensors: add enable and poll_delay APIs for sensor class driver

The sensor class driver will create the "enable" and "poll_delay"
handles in the "/sys/class/sensors/xxx/" for each sensor driver.
The userspace code could easily find the control interface by
searching the "/sys/class/sensors/" folder.

Change-Id: Ia18f062262e4b78db7d7de87acb49dfcf485063b
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>