arm64: dts: socfpga: Use generic "ngpios" rather than "snps,nr-gpios"

This is to remove similar errors as below:

OF: /.../gpio-port@0: could not find phandle

Commit 7569486d ("gpio: dwapb: Add ngpios DT-property support")
explained the reason of above errors well and added the generic
"ngpios" property, let's use it.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dinh Nguyen <dinguyen@kernel.org>