tbs2910: Convert to DM_SERIAL

Conversion to DM_SERIAL is mandatory.

Select DM_SERIAL and add a imx6q-tbs2910-u-boot.dtsi file
that describes 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>
Tested-by: Soeren Moch <smoch@web.de>