ARM: s32v234evb: Set CONFIG_DM & CONFIG_DM_SERIAL in Kconfig

These symbols are declared in Kconfig, so it's wrong to set them in
header files.

Note that this is not size-neutral - some 'default y' options will now
get turned on by Kconfig, such as CONFIG_CMD_DM=y and CONFIG_DM_STDIO=y.

Signed-off-by: Tuomas Tynkkynen <tuomas@tuxera.com>