ARM: dts: socfpga: rename gpio nodes

Since the Synopsys GPIO IP can support multiple ports of varying widths, it
would make more sense to have the GPIO node DTS entry as this:

gpio0: gpio@ff708000{
	porta{
	};
};

Also, this is documented in the snps-dwapb-gpio.txt.

Suggested-by: Doug Anderson <dianders@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Dinh Nguyen <dinguyen@opensource.altera.com>