gpio: xilinx: Switch to use bitmap APIs

It seems that Xilinx GPIO driver operates with bit arrays longer than 32 and
thus can leverage bitmap APIs for that. It makes code better to understand.

The ->probe() function is modified to try read properties for both channels
since is_dual check makes only sense for the amount of pins used for the second
channel. On top of that kzalloc() guarantees zero initial values for the fields
in the private data structure, hence drop unneeded conditionals and assignments.

The change is inspired by Syed Nayyar Waris' ideas about bitmap API extension.

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>