dt: amd-seattle: add a description of the CPUs and caches

Add a DT description of the CPU and cache hierarchy as found on
the AMD Seattle SOC. Given the tight coupling of the PMU with
the CPUs, move the PMU node into the cpu .dtsi file as well, and
add the missing affinity description.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>