MIPS: JZ4740: Add prom support

Add support for initializing arcs_cmdline on JZ4740 based machines and
provides a prom_putchar implementation.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/1404/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>