dt: amd-seattle: fix PCIe legacy interrupt routing

The AMD Seattle SOC can be configured to expose up to 3 PCIe root
ports, each of which is wired to 4 dedicated SPI wired interrupts
for legacy INTx support. Update the SOC DT description to reflect
this.

Fix a stale comment about the size of the MMIO64 resource window
while at it.

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