iio: exyno-adc: use syscon for PMU register access

This patch updates the IIO based ADC driver to use syscon and regmap
APIs to access and use PMU registers instead of remapping the PMU
registers in the driver.

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