fbdev/simplefb: Support memory-region property

The simple-framebuffer bindings specify that the "memory-region"
property can be used as an alternative to the "reg" property to define
the framebuffer memory used by the display hardware. Implement support
for this in the simplefb driver.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231101172017.3872242-2-thierry.reding@gmail.com