irqchip/gic-v3-its: Remove artificial dependency on PCI

The GICv3 ITS doesn't really depend on PCI. Only the PCI/MSI
part of it does, and there is no reason not to blow away most
of the irqchip stack because PCI is not selected (though not
selecting PCI seem to be asking for punishment, but hey...).

So let's split the PCI-specific part from the ITS in the Kconfig
file, and let's make that part depend on PCI. Architecture specific
hacks (arch/arm{,64}/Kconfig) will be addressed in a separate patch.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>