kconfig: Hide the DesignWare I2C driver on unsupported platforms

When users are configuring nRF applications they are given the option
to enable the DesignWare I2C driver. But they should not be given this
option because nRF SoCs do not have DesignWare I2C HW.

This commit hides the driver by default by introducing the config
option HAS_I2C_DW.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>