ARM: imx_v6_v7_defconfig: Add missing config for DART-MX6 SoM

This patch adds the missing configs for the DART-MX6 SoM support :
- SERDEV bluetooth driver + SERIAL_DEV_BUS configs
- WL18XX driver
- DEFAULT_ON Led Trigger

Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>