PCI: keystone: Prevent ARM32 specific code to be compiled for ARM64

hook_fault_code() is an ARM32 specific API for hooking into data abort.

AM65X platforms (that integrate ARM v8 cores and select CONFIG_ARM64 as
arch) rely on pci-keystone.c but on them the enumeration of a
non-present BDF does not trigger a bus error, so the fixup exception
provided by calling hook_fault_code() is not needed and can be guarded
with CONFIG_ARM.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
[lorenzo.pieralisi@arm.com: commit log]
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>