i2c: designware: Allow build Baytrail semaphore support when IOSF_MBI=m

I believe i2c-designware-baytrail.c doesn't have strict dependency that
Intel SoC IOSF Sideband support must be always built-in in order to be
able to compile support for Intel Baytrail I2C bus sharing HW semaphore.

Redefine build dependencies so that CONFIG_IOSF_MBI=y is required only
when CONFIG_I2C_DESIGNWARE_PLATFORM is built-in.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: David Box <david.e.box@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>