riscv: dts: thead: Adjust TH1520 GPIO labels

Adjust labels for the TH1520 GPIO controllers such that GPIOs can be
referenced by the names used by the documentation. Eg.

GPIO0_X  -> <&gpio0 X Y>
GPIO1_X  -> <&gpio1 X Y>
GPIO2_X  -> <&gpio2 X Y>
GPIO3_X  -> <&gpio3 X Y>
GPIO4_X  -> <&gpio4 X Y>
AOGPIO_X -> <&aogpio X Y>

Remove labels for the parent GPIO devices that shouldn't need to be
referenced.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Thomas Bonnefille <thomas.bonnefille@bootlin.com>
Signed-off-by: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Signed-off-by: Drew Fustini <dfustini@tenstorrent.com>