dtoverlays: Add inverted override property to ssd1306-spi

The new ssd130x DRM driver supports both SSD1306 I2C and SPI panels and is
compatible with the ssd1307fb driver bindings. So the "solomon,com-invdir"
DT property (to invert the COM pin scan dir) can also be used for SPI now.

This allows to configure panels whose scan direction needs to be inverted.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>