drm/panel-simple: Read panel orientation

The panel orientation needs to parsed from a device-tree and assigned to
the panel's connector in order to make orientation property available to
userspace. That's what this patch does for the panel-simple driver.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200813215609.28643-5-digetx@gmail.com