board: bsh: imx8mn_bsh_smm_s2/s2pro: enable DM_SERIAL

board: bsh: imx8mn_bsh_smm_s2/s2pro: enable DM_SERIAL

Enable DM_SERIAL for both U_Boot and the SPL. The uart4 and its pinmux
are already marked with u-boot,dm-spl but we need to move the call to
preloader_console_init() after spl_init() to avoid a board hang
as dm can't be used until after spl_init().

Remove the manual config of the UART pinmux now that it is no longer
needed.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>
Cc: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Tested-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>