ARM: kconfig: allow PCI support to be selected with ARCH_MULTIPLATFORM

When targetting ARCH_MULTIPLATFORM, we may include support for SoCs with
PCI-capable devices (e.g. mach-virt with virtio-pci).

This patch allows PCI support to be selected for these SoCs by selecting
CONFIG_MIGHT_HAVE_PCI when CONFIG_ARCH_MULTIPLATFORM=y and removes the
individual selections from multi-platform enabled SoCs.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Olof Johansson <olof@lixom.net>