arm64: dts: sparx5: Add basic cpu support

This adds the basic DT structure for the Microchip Sparx5 SoC, and the
reference boards, pcb125, pcb134 and pcb135. The two latter have a
NAND vs a eMMC centric variant (as a mount option).

Link: https://lore.kernel.org/r/20200615133242.24911-4-lars.povlsen@microchip.com
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>