lib: Makefile: build fdtdec_common.c when OF_LIBFDT selected

When build SPL_OF_PLATDATA on i.MX6, meet issue the fdtdec_get_int
not defined, however fdtdec.c will use fdtdec_get_int, so let's
compile fdtdec_common.c when OF_LIBFDT selected.

Since there is also SPL_OF_LIBFDT, so need to use
CONFIG_$(SPL_TPL_)OF_LIBFDT.

Signed-off-by: Peng Fan <peng.fan@nxp.com>