i2c: designware: Add Baytrail sem config DW I2C platform dependency

Currently Intel Baytrail I2C semaphore is a feature of the DW APB I2C
platform driver. It's a bit confusing to see it's config in the menu at
some separated place with no reference to the platform code. Let's move the
config definition to be below the I2C_DESIGNWARE_PLATFORM config and mark
it with "depends on I2C_DESIGNWARE_PLATFORM" statement. By doing so the
config menu will display the feature right below the DW I2C platform
driver item and will indent it to the right so signifying its belonging.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>