ARM: mxs: Change duart device to use amba-pl011

The mxs duart is actually an amba-pl011 device. This commit changes
the duart device code to dynamically allocate amba-pl011 device,
so that drivers/serial/amba-pl011.c can be used on mxs.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>