OMAP: UART: Add sysfs interface for adjusting UART sleep timeout

This patch makes it possible to change uart sleep timeout. New sysfs
entry is added (/sys/devices/platform/serial8250.<uart>/sleep_timeout)
Writing zero will disable the timeout feature and prevent UART clocks
from being disabled.

Also default timeout is increased to 5 second to make serial console
more usable.

Original patch was written by Tero Kristo.

Cc: Tero Kristo <Tero.Kristo@nokia.com>
Signed-off-by: Jouni Hogander <jouni.hogander@nokia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>