arch: powerpc: Remove unused dts frequency fixup for lbc_clk

lbc_clk is used to fixup dts as "bus frequency".
It is not being used by Linux IFC and eLBC driver.

So remove unused "bus frqeuency" fix-up of devicre tree.

Signed-off-by: Prabhakar Kushwaha <prabhakar.kushwaha@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>