PCI: imx6: Fix imprecise abort handler

An imprecise abort is triggered when a port behind a switch is accessed
and no device is present.  At enumeration, imprecise aborts are not enabled
thus this ends up getting deferred until the kernel has completed init.  At
that point we must not adjust PC - the handler must do nothing, but a
handler must exist.

This fixes random crashes that occur right after freeing init.

Tested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>