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>