arm: mvebu: Synchronize armada-38x.dtsi with Linux v5.20

* Replace skeleton.dtsi by explicit #address-cells / #size-cells
* Add sdramc@1400 and phy@18300 nodes
* Remove (unused) timeout-ms i2c properties
* Fix compatible string for UARTs
* Add interrupts properties for watchdog

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>