OMAP: DSS2: Implement dsi_mux_pads for OMAP4

Implement dsi_mux_pads for OMAP4. On enable the function enables the DSI
pins and disables pull down. On disable the function disables the pins
and enables pull down.

It is unclear from the TRM whether the pull down is active if the pins
are disabled, so this implementation may leave the pins floating when
the DSI device is disabled.

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