dts: xea: Add u-boot specific 'type' property to duart

dts: xea: Add u-boot specific 'type' property to duart

The DM_SERIAL implicitly requires CONFIG_PL01X_SERIAL, which
allows support for both serial IP block versions (i.e. PL011 and
PL010).

The decision about used IP block is based on the compatible string,
when DM is used.

In the XEA, the OF_PLATDATA is used to allow usage of serial driver in
the SPL (as the size of SPL is crucial). In this case one cannot extract
the type of IP block from .data field (corresponding to compatible) and
it must be explicitly read at probe from dtoc generated, u-boot specific
property.

Signed-off-by: Lukasz Majewski <lukma@denx.de>