fbdev: da8xx: add support for a regulator

We want to remove the hacky platform data callback for power control.
Add a regulator to the driver data and enable/disable it next to
the current panel_power_ctrl() calls. We will use it in subsequent
patch on da850-evm.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>