Makefile: Correct TPL rule for OF_REAL

Correct an error in the tpl-dtb parameter to binman. At present the TPL
rule follows SPL but this is not correct, if TPL uses of-platdata, for
example.

Fixes: f99cbe4e ("fdt: Update Makefile rules with the new OF_REAL Kconfig")
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>