config: xea: Enable DM_SERIAL for the XEA - single binary (SB) u-boot

The single binary version of u-boot for XEA board is used to debrick and
factory programming.

The produced u-boot.sb is a single file, which allows having fully
operational u-boot prompt loaded with imx287 ROM.

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

configs/imx28_xea_sb_defconfig

@@ -94,7 +94,9 @@ CONFIG_PINCTRL_MXS=y
 CONFIG_DM_REGULATOR=y
 CONFIG_DM_REGULATOR_FIXED=y
 CONFIG_DM_REGULATOR_GPIO=y
+CONFIG_SPECIFY_CONSOLE_INDEX=y
 CONFIG_CONS_INDEX=0
+CONFIG_DM_SERIAL=y
 CONFIG_SPI=y
 CONFIG_DM_SPI=y
 CONFIG_MXS_SPI=y