MIPS: JZ4740: Forward declare struct uart_port in header.

As suggested by Geert Uytterhoeven <geert@linux-m68k.org>.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: Lars-Peter Clausen <lars@metafoo.de>