video: fbdev: clps711x-fb: Use syscon_regmap_lookup_by_phandle

Since version 5.13, the standard syscon bindings have been added
to all clps711x DT nodes, so we can now use the more general
syscon_regmap_lookup_by_phandle function to get the syscon pointer.

Signed-off-by: Alexander Shiyan <eagle.alexander923@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>