dt-bindings: serial: uartlite: drop $ref for -bits property

dt-bindings: serial: uartlite: drop $ref for -bits property

Fixes warning:
Documentation/devicetree/bindings/serial/xlnx,opb-uartlite.yaml: properties:xlnx,data-bits: {'const': '$ref'} is not allowed for '$ref'
	hint: Standard unit suffix properties don't need a type $ref
	from schema $id: http://devicetree.org/meta-schemas/core.yaml#

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: David Heidelberg <david@ixit.cz>
Link: https://lore.kernel.org/r/20211008234312.61688-1-david@ixit.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>