dt-bindings: display: arm: versatile: Pass the sysreg unit name

Pass the sysreg unit name to fix the following warning seen with
'make dt_binding_check':

Warning (unit_address_vs_reg): /example-0/sysreg: node has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Link: https://lore.kernel.org/r/20200629215500.18037-1-festevam@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>