powerpc/4xx: Add check_link to struct ppc4xx_pciex_hwops

All current pcie controllers unconditionally use SDR to check the link and
poll for reset.  Refactor the code to include device reset in the
port_init_hw() op and add a new check_link() op.

This will make room fro new controllers that do not use SDR for these
operations.

Tested on 460ex.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>