ARM: SAMSUNG: use regulator VDD for ADC

This patch allows the Samsung ADC driver to enable VDD regulator
at probe and resume and to disable at exit and suspend.
In a platform where ADC's VDD regulator is not "always-on", this
control is required although this patch does not provide fine-grained
power control (turning on the regulator only when being accessed).

However, if VDD regulator ("vdd" for the adc device) is not provided,
the regulator control will not be activated because there are platforms
that do not provide regulator for ADC device.

arch_initcall has been modified to module_init in order to allow
regulators to be available at probe.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>