Documentation: dt-bindings: update exynos-adc.txt with syscon handle

This patch updates the DT bindings for ADC in exynos-adc.txt with the
syscon phandle to the ADC nodes.

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
To: devicetree@vger.kernel.org
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>