of: Move simple-framebuffer device handling from simplefb to of

This code is required for both simplefb and simpledrm, so let's move it
into the OF core instead of having it as an ad-hoc initcall in the
drivers.

Signed-off-by: Hector Martin <marcan@marcan.st>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211212062407.138309-2-marcan@marcan.st