OMAPDSS: Add common PLL code

OMAP DSS currently contains two different PLLs: DSI PLL (Type A PLL) and
HDMI PLL (Type B PLL). When DRA7 support is added, we will also support
Video PLLs (Type A).

The driver currently handles all PLLs totally separately. This patch
adds common DSS PLL code, which

a) lets us have common code for the PLLs
b) lets the users of the PLLs use a common API, instead of DSI API or
   HDMI API.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>