arm64: dts: librem5: Drop assigned-clocks from SAI2

IMX8MQ_AUDIO_PLL1 and IMX8MQ_AUDIO_PLL2 are setup to the same rates
right on the clock controller.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>