dt-bindings: display: ssd1307fb: Extend schema for SPI controllers

dt-bindings: display: ssd1307fb: Extend schema for SPI controllers

The Solomon SSD130x OLED displays can either have an I2C or SPI interface,
add to the schema the properties and examples for OLED devices under SPI.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220419214824.335075-3-javierm@redhat.com