dt-bindings: display/ti: Add optional property to set memory bandwidth limit

max-memory-bandwidth can be used to specify the maximum bandwidth dispc
can use when reading display data from main memory.

In some SoC (am437x for example) we have memory bandwidth limitation
which causes underflow in the display subsystem.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>