Add fifo-merge support. This is done mainly in four functions:
mgr_enable/disable and ovl_enable/disable. These are the functions where
overlays are taken into and out of active use.

The process to enable and disable fifo-merge is not simple. We need to
do it in steps, waiting in between for certain settings to be taken into
use, and continuing after that. The reason for this is that fifo-merge
is a common thing for all managers/overlays, and its use must be
synchronized.

As an example, when we disable an overlay, we first set the overlay as
disabled, then wait until the overlay is actually disabled in the HW,
and only after that we may re-configure the fifos, possibly taking
fifo-merge into use.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>