dts: nxp: nxp_ke1xf: Make LPO fixed-clock its own node

Pull out the LPO fixed-clock that is part of the PMC hardware block as
its own child node of the PMC block.  This is because the PMC could have
its own driver associated with it that is seperate from the LPO clk.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>