serdev: Add ACPI support

This patch allows SerDev module to manage serial devices declared as
attached to an UART in ACPI table.

acpi_serdev_add_device() callback will only take into account entries
without enumerated flag set. This flags is set for all entries during
ACPI scan, except for SPI and I2C serial devices, and for UART with
2nd patch in the series.

Check if a serdev device as been allocated during acpi_walk_namespace()
to prevent serdev controller registration instead of the tty-class device.

Signed-off-by: Frédéric Danis <frederic.danis.oss@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Reviewed-by: Johan Hovold <johan@kernel.org>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>