mfd: intel_quark_i2c_gpio: Reuse BAR definitions for MFD cell indexing

It's convenient and less error prone to use definitions to address
different cells in an array. For this purpose we may reuse existing
BAR definitions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>