gpio: tb10x: Create local helper variables

Create a local struct device *dev helper variable to make the code
easier to read.

Most GPIO drivers use "np" (node pointer) rather than "dn" (device node)
to point to the device tree node. Let's follow this convention.

Cc: linux-snps-arc@lists.infradead.org
Acked-by: Christian Ruppert <christian.ruppert@alitech.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>