sensors: add enable and poll_delay handle for bmp18x sensor drver

Add enable and poll_delay handle functions for the bmp18x pressure
driver. The system will create the general enable and poll_delay handles
in the "/sys/class/sensors/xxx/" for each sensor drvier which is using
the sensor class driver.

Change-Id: I1e24cbc6a63c5e1e5eb4db66049bd7be85b81352
Signed-off-by: Jie Cheng <rockiec@codeaurora.org>