gpio: mmio: Add compatible for opencores GPIO

gpio: mmio: Add compatible for opencores GPIO

On FPGA Development boards with GPIOs the OpenRISC architecture uses the
opencores gpio verilog rtl.  This is compatible with the gpio-mmio.  Add
the compatible string to allow probing this driver from the devicetree.

Link: https://opencores.org/projects/gpio
Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/20260115151014.3956805-3-shorne@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>