QE: Add uqe_serial document to bindings

Add uqe_serial document to
Documentation/devicetree/bindings/powerpc/fsl/cpm_qe/uqe_serial.txt

Signed-off-by: Zhao Qiang <qiang.zhao@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Scott Wood <oss@buserror.net>