regulator: anatop: make regulator name property required

We actually can't allow the missing of the regualor name, thus update
the binding doc to make regulator-name property to be required.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Mark Brown <broonie@kernel.org>