ACPI / LPSS: not using UART RTS override with Auto Flow Control

Adding a check for UART Auto Flow Control feature and only
enabling the RTS override when it's not supported. RTS
override is not needed when Auto Flow Control is used and
they shouldn't be used together.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>