drm/exynos: use regmap interface to set hdmiphy control bit in pmu

Exynos drm hdmi driver used to get dummy hdmiphy clock to
control the PMU bit for hdmiphy. This bit needs to be set
before setting any resolution to hdmi hardware. This was
handled using dummy hdmiphy clock which is removed here.

PMU is already defined as system controller for exynos
SoCs. Hdmi driver is modified to control the phy enable bit
inside PMU using regmap interfaces.

Devicetree binding document for hdmi is also updated.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>