samples: mesh: boards: nrf52: modifications as per 3.3.2.2.3

Added extra case under state binding's switch statement for
Lightness i.e DELTA_LEVEL. Also upgrade binding between

1. root element's LEVEL state & Light Lightness Actual state.
2. Light Lightness Linear state & Light Lightness Actual state.
3. Light CTL lightness state & Light Lightness Actual state

This is as per Bluetooth Mesh Model Specification 3.3.2.2.3.

Signed-off-by: Vikrant More <vikrant8051@gmail.com>