gpio: Remove unused local OF node pointers

After the commit 448cf905 ("gpio: Get rid of duplicate of_node
assignment in the drivers") the OF node local pointers become unused.
Remove them for good and make compiler happy about.

Fixes: 448cf905 ("gpio: Get rid of duplicate of_node assignment in the drivers")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Bart: removed unrelated change in gpio-brcmstb]
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>