dts: bindings: clocks: Add binding for F412 PLL I2S

Provide dts binding for F412 PLL I2S.
This I2S dedicated PLL is fully configurable and take same
input as Main PLL

Only one output clock (PLLR) is supported for now.
This PLL could be found on STM32F412 and F413 parts for instance.

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