dt-bindings: gpio: sifive,gpio: Group interrupt tuples

To improve human readability and enable automatic validation, the tuples
in "interrupts" properties should be grouped using angle brackets.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>