dt-bindings: gpio-mmio: Add compatible string for opencores,gpio

dt-bindings: gpio-mmio: Add compatible string for opencores,gpio

In FPGA Development boards with GPIOs we use the opencores gpio verilog
rtl.  This is compatible with the gpio-mmio.  Add the compatible string
to allow as below.

Example:

        gpio0: gpio@91000000 {
                compatible = "opencores,gpio", "brcm,bcm6345-gpio";
                reg = <0x91000000 0x1>, <0x91000001 0x1>;
                reg-names = "dat", "dirout";
                gpio-controller;
                #gpio-cells = <2>;
                status = "okay";
        };

Link: https://opencores.org/projects/gpio
Signed-off-by: Stafford Horne <shorne@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20251217080843.70621-2-shorne@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>