MIPS: BCM47xx: Swap serial console if ttyS1 was specified.

Some devices like the Netgear WGT634U are using ttyS1 for default console
output. We should switch to that console if it was given in the kernel_args
parameters.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
To: linux-mips@linux-mips.org
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Patchwork: https://patchwork.linux-mips.org/patch/1848/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>