pinctrl: at91-pio4: convert to dev_read_prop

Use dev_read_prop instead of using the fdt_read_property which
reads from the GD struct's fdt.
This way the node is accessed via the device config instead of the
global struct, which makes code more portable and GD independent.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>