arm64: dts: keembay: Add device tree for Keem Bay SoC

Add initial device tree for Intel Movidius SoC code-named Keem Bay.

This initial DT includes nodes for Cortex-A53 cores, UARTs, GIC, PSCI,
and PMU.

Link: https://lore.kernel.org/r/20200717090414.313530-5-daniele.alessandrelli@linux.intel.com
Reviewed-by: Dinh Nguyen <dinguyen@kernel.org>
Signed-off-by: Daniele Alessandrelli <daniele.alessandrelli@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>