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 <robh+dt@kernel.org>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: Benoît Cousson <bcousson@baylibre.com>
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: NRoger Quadros <rogerq@ti.com>
Signed-off-by: NJyri Sarha <jsarha@ti.com>
Signed-off-by: NBartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>