dt-bindings: display: ssd1307fb: Convert to json-schema

Convert the Solomon SSD1307 Framebuffer Device Tree binding
documentation to json-schema.

Fix the spelling of the "pwms" property.
Document default values.
Make properties with default values not required.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714145122.2530176-1-geert@linux-m68k.org