When booting Barebox in the HW environment where PCIe core has been
used but not properly shut down, writing to PCIE_PL_PFLR in
imx6_pcie_assert_core_reset would cause data abort exception.

The problem can be easily reproduced on a i.MX6 based board with PCIe
slot populated with some device by doing:

> bootm -f -e 0x1050 <your board's>.img

Ignoring this exception seem to allow PCIe core to successfully
initialize and enumerate devices properly. This is also how Linux
Kernel version of the driver handles this situation -- it installs
dummy no-op abort handler at the beginning of device's probing.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>