ARM: dts: imx50-kobo-aura: switch to enable-gpios

The recommended name for enable GPIOs property in regulator-gpio is
"enable-gpios".  This is also required by bindings:

  imx50-kobo-aura.dtb: gpio-regulator: Unevaluated properties are not allowed ('enable-gpio' was unexpected)

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>