Samples: Fix XEC clock control sample code for MEC172x using new GPIOs.

The XEC clock control driver sample code uses enum indexes to offset to
the desired GPIO CTRL register of the package.  These enums are only
used in this example code and are being removed in favor of calculated
indexing into the specific GPIO register memory map.

Signed-off-by: Jeff Daly <jeffd@silicom-usa.com>