i2c: designware: Find bus speed from ACPI

Fast mode is the default speed of i2c-designware which can be overridden
by platform data or by "clock-frequency" device property. Even though
the ACPI 5.1 can pass device properties via _DSD method, shipping systems
define the connection speed between I2C host and each slave in their
I2cSerialBus resources. Which means speed is not defined per bus but per
slave.

As there is now support in i2c-core to find the bus speed from ACPI use
that to set up the bus speed prior registering the I2C adapter.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>