fbdev: simplefb: Use of_reserved_mem_region_to_resource() for "memory-region"

Use the newly added of_reserved_mem_region_to_resource() function to
handle "memory-region" properties.

The error handling is a bit different. "memory-region" is optional, so
failed lookup is not an error. But then an error in
of_address_to_resource() is treated as an error. However, that
distinction is not really important. Either the region is available
and usable or it is not. So now, it is just
of_reserved_mem_region_to_resource() which is checked for an error.

Signed-off-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Helge Deller <deller@gmx.de>