fbdev/ssd1307fb: add support to enable VBAT

fbdev/ssd1307fb: add support to enable VBAT

SSD1306 needs VBAT when it is wired in charge pump configuration. This
patch adds support to the driver to enable VBAT regulator at init time.

Cc: Rob Herring <[email protected]>
Cc: Maxime Ripard <[email protected]>
Cc: Benoît Cousson <[email protected]>
Signed-off-by: Tomi Valkeinen <[email protected]>
Reviewed-by: Roger Quadros <[email protected]>
Signed-off-by: Jyri Sarha <[email protected]>
Signed-off-by: Bartlomiej Zolnierkiewicz <[email protected]>