serial: Add missing TPL_SYS_NS16550_SERIAL symbol

On PowerPC platforms with TPL enabled and SPL_SYS_NS16550_SERIAL
enabled, today this builds under TPL as well due to how $(XPL_) is
defined. Add the TPL_SYS_NS16550_SERIAL itself for consistency and
clarity.

Signed-off-by: Tom Rini <trini@konsulko.com>
1 file changed