serial: 8250_dw: don't limit DMA support to ACPI

It should be available for DT users as well. This does not
enable DMA by default except with ACPI. DT users can enable
DMA based on a property.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>