drivers: video: Add get_volatile_ctrl driver's API

Add get_volatile_ctrl() driver's API to retrieve the current value of a
control marked as volatile, e.g. gain, exposure. This function triggers
a hardware register reading instead of returning a cached value to ensure
that users always get a fresh value which is constantly updated by the HW.

Note that the driver is responsible for marking a control as volatile by
setting VIDEO_CTRL_FLAG_VOLATILE when registering a control because not
all hardwares work the same way for the same control.

Signed-off-by: Phi Bang Nguyen <phibang.nguyen@nxp.com>