gpio: xilinx: Introduce xgpio_read_chan() / xgpio_write_chan()

With the new helpers, i.e. xgpio_read_chan() / xgpio_write_chan(),
the code is easier to read and maintain. No functional changes
intended.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Tested-by: Neeli Srinivas <sneeli@xilinx.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>