dt-bindings: soc: ti: pruss: Add dma-coherent property

Update the PRUSS schema file to include the dma-coherent property
that indicates the coherency of the IP. The PRUSS IPs on 66AK2G
SoCs do use this property.

The new added dma-coherent property is a required property _only_
for 66AK2G SoCs and is not required/applicable for other SoCs, so
the binding is backward compatible for other SoCs. This update is
being done before the corresponding dts nodes can be added for 66AK2G
SoCs.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>