OMAP: DSS2: Implement OMAP4 DSS fclk support

Add dss.dpll4_m4_ck (DSS FCLK) initialization for OMAP4. This is used
to compute the pixel clock for DPI interface and also to reconfigure
the DSS FCLK to the desired rate, corresponding to the rate computed
for pixel clock.

Adding these cpu_is_44xx() checks are meant to be temporary, until a
cleaner implementation to manage these checks are added. Currently this
is needed to get DVI display running on OMAP4 PandaBoard

Signed-off-by: Raghuveer Murthy <raghuveer.murthy@ti.com>
[tomi.valkeinen@ti.com: minor changes due to conflicts]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>