iio: adc: Add dt support for turning on the phy in exynos-adc

Without this change the exynos adc controller needed to have its phy
enabled in some out-of-driver C code.  Add support for specifying the
phy enable register by listing it in the reg list.

Signed-off-by: Doug Anderson <dianders@chromium.org>
Tested-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>