pci: pcie_layerscape_fixup_common: lx2_board_fix_fdt can be static

To avoid W=1 build warnings, declare this function as static, since it
is not used outside of this translation module.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>