dt-bindings: mfd: mc13xxx: Add the unit address to sysled

As the 'reg' property is mandatory in the subnodes, improve the
example by adding the unit address to the sysled node.

This prevents the following build warning with W=1:

Node /soc/aips@70000000/spba@70000000/ecspi@70010000/pmic@0/leds/sysled0 has a reg or ranges property, but no unit name

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>