MIPS: Octeon: Serial port fixes for OCTEON simulator.

For the simulator, fake a slow clock to get fast output.

In prom_putchar we have to mask the value so the simulator doesn't
ASSERT when printing non-ASCII characters.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/1255/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>