GPIO: xilinx: Simplify driver probe function

Simplification is done by using OF helper function
which increase readability of code and remove
(if (var) var = be32_to_cpup;) assignment.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>