include: dt-bindings: F4 clocks: Add PLLI2S as source clock

Add PLLI2S R as I2S source clock.
For now I2S_CKIN (fixed clock) is not supported.

This change only consider F401 and compatible PLL I2S implementations.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>