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>
diff --git a/configs/tbs2910_defconfig b/configs/tbs2910_defconfig
index 993997b..60a8160 100644
--- a/configs/tbs2910_defconfig
+++ b/configs/tbs2910_defconfig
@@ -25,7 +25,6 @@
 CONFIG_PREBOOT="echo PCI:; pci enum; pci 1; usb start; if hdmidet; then run set_con_hdmi; else run set_con_serial; fi"
 CONFIG_DEFAULT_FDT_FILE="imx6q-tbs2910.dtb"
 CONFIG_PRE_CONSOLE_BUFFER=y
-CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Matrix U-Boot> "
 # CONFIG_CMD_BDI is not set
@@ -83,6 +82,7 @@
 CONFIG_PINCTRL_IMX6=y
 CONFIG_DM_RTC=y
 CONFIG_RTC_DS1307=y
+CONFIG_DM_SERIAL=y
 CONFIG_MXC_UART=y
 CONFIG_DM_THERMAL=y
 CONFIG_IMX_THERMAL=y