mod_sensor: Add enable/disable of sensors api

This patch adds two functions to the mod_sensor api to enable
or disable the sensor. Specifically, these functions call
functions within the driver api (if provided) to set the
"disabled" flag to true or false.

Change-Id: I93d877d4c444bc519bc6ca0df446bd00edba507b
Signed-off-by: Andrew Graham <andrew.graham@arm.com>