platform/x86: i2c-multi-instantiate: Count I2cSerialBus() resources

Instead of relying on hard coded and thus expected number of I2C clients,
count the real amount provided by firmware.

This allows to support non-fixed amount of the slaves.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>