arm64: defconfig: Enable PCI generic host bridge by default

Now that pci-host-generic can be used under arm64, enable it by
default so that SBSA compliant systems can use it.

Cc: Will Deacon <will.deacon@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>