Drivers: ssd1306: Add use_internal_iref DTS option

Necessary for supporting for EastRising 0.42 OLED display/board.

Some boards don't have external Iref set up. This is probably done in an
effort to save on component cost. This command is only documented in the
V1.1 revision of the SSD1306 datasheet.

See issue https://github.com/olikraus/u8g2/issues/1047



Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>