omap: Disable serial port autoidle by default

Currently the omap serial clocks are autoidled after 5 seconds.
However, this causes lost characters on the serial ports. As this
is considered non-standard behaviour for Linux, disable the timeout.

Note that this will also cause blocking of any deeper omap sleep
states.

To enable the autoidling of the serial ports, do something like
this for each serial port:

# echo 5 > /sys/devices/platform/serial8250.0/sleep_timeout
# echo 5 > /sys/devices/platform/serial8250.1/sleep_timeout
...

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>