board: argonkey: add LSM6DSL configuration in Kconfig.defconfig

Use the named choices feature introduced by PR #6966
and configure LSM6DSL on ArgonKey board.
The two named choices introduced in LSM6DSL are:

    - LSM6DSL_BUS_TYPE (default LSM6DSL_I2C)
    - LSM6DSL_TRIGGER_MODE (default LSM6DSL_TRIGGER_NONE)

Signed-off-by: Armando Visconti <armando.visconti@st.com>