arm64: amd-seattle: Adding device tree for AMD Seattle platform

Initial revision of device tree for AMD Seattle Development platform.

Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Signed-off-by: Thomas Lendacky <Thomas.Lendacky@amd.com>
Signed-off-by: Joel Schopp <Joel.Schopp@amd.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>