serial: 8250: Add OF support for Xscale variant

This adds support for device tree probing for the Intel
Xscale 8250 variant needed to support device tree on
the Intel IXP4xx platforms.

Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>