ARM: imx6q_logic: Enable DM_SERIAL

In order to call preloader_console_init from board_init_f when
DM_SERIAL is enabled, it neesd to call spl_early_init() to get early access
to DM and device tree.

This patch calls spl_early_init just before preloader_console_init()
and enables DM_SERIAL and SPL_DM_SERIAL.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>