warp7: Remove UART initialization code

With DM_SERIAL selected, it is no longer needed board code to
initialize the UART.

Describe the nodes that require dm-pre-reloc, which allows
the DM model to configure the UART pinctrl early.

Remove the now unneeded board UART initialization.

Signed-off-by: Fabio Estevam <festevam@denx.de>