do no longer rename amba-pl010 serial devices names

Back in the days when we could not override /etc/inittab, it made a lot
of sense to rename ttyAM to ttyS since this was the only supported
serial device(s), we can now just put in an appropriate /etc/inittab file
now.

Signed-off-by: Florian Fainelli <florian@openwrt.org>

SVN-Revision: 34469