OMAP4: DSS2: Register configuration changes for DSI

The following changes have changed from OMAP3 to OMAP4 DSI:
-The register field DSI_PLL_FREQSEL in DSI_PLL_CONFIGURATION2 has been removed.
-DCS_CMD_ENABLE and DCS_CMD_CODE bits have been moved from DSI_CTRL to
DSI_VC_CTRLi, hence the control of the bits is available per VC.
-DSI LDO powergood notification doesn't work on OMAP4. This is mentioned in
OMAP4 errata revision 1.8(Errata 1.76).
-OCP_WIDTH register field is included in DSI_VC_CTRL.
-The SCP clock is also required to access DSI PLL registers

Introduce dss features for these changes so that DSI runs on both OMAP3 and
OMAP4.

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