ARM: dts: stm32: remove sai kernel clock on stm32mp15xx-dkx

Remove clocks property in the SAI2A subblock node as this property
is already defined in the SoC DT.
Keep only this property when the master clock is added in the node.

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>