x86: apl: Adjust how the UART gets its platform data

x86: apl: Adjust how the UART gets its platform data

At present this driver calls malloc() to start a new platform data
structure, fills it in and tells driver model to use it.

We want to avoid malloc, particularly with the new version of of-platdata.
Create a new struct which encompasses both the dtd struct and the ns16550
one, to avoid this. Unfortunately we must copy the data into the right
place for the ns16550 driver. Add some comments about this.

Signed-off-by: Simon Glass <sjg@chromium.org>