dt-bindings: Clean-up regulator '-supply' schemas

dt-bindings: Clean-up regulator '-supply' schemas

Regulator '*-supply' properties are always a single phandle, so
'maxItems: 1' or a $ref is not necessary. All that's needed is either
'true' or an optional 'description'. Following this clean-up, the
meta-schema will enforce this pattern.

There's one case in tree with 'innolux,n156bge-l21' having 2 phandles.
This appears to be a mistake or abuse of simple-panel as it's 2 different
voltage rails connected to 'power-supply'.

Cc: Neil Armstrong <narmstrong@baylibre.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Kishon Vijay Abraham I <kishon@ti.com>
Cc: Liam Girdwood <lgirdwood@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: linux-iio@vger.kernel.org
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com> # for iio
Signed-off-by: Rob Herring <robh@kernel.org>