arm64: dts: sparx5: Add SGPIO devices

This adds SGPIO devices for the Sparx5 SoC and configures it for the
applicable reference boards.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20201113145151.68900-4-lars.povlsen@microchip.com