dt-bindings: clk: at91: add an I2S mux clock

The I2S mux clock can be used to select the I2S input clock. The
available parents are the peripheral and the generated clocks.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>