LF-4147-1 arm64: dts: imx8ulp: increase the clock speed of LPSPI

LPSPI transfer max speed is half of the root clock.
Increase the root clock speed to support faster data transmission.

Signed-off-by: Clark Wang <xiaoning.wang@nxp.com>
Reviewed-by: Haibo Chen <haibo.chen@nxp.com>