OMAPDSS: Add functions for external control of PLL

Add functions which configure the control module register
CTRL_CORE_DSS_PLL_CONTROL found in DRA7xx SoCs. This register configures
whether the PLL registers are accessed internally by DSS, or externally
using OCP2SCP interface. They also configure muxes which route the PLL
output to a particular LCD overlay manager within DSS.

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