83xx/pcie: fix build error for 83xx pcie

Fix the following build error caused by patch "powerpc/pcie: add PCIe
version 3.x support":

pcie.c:302:34: error: 'PCI_LTSSM' undeclared (first use in this function)
pcie.c:303:15: error: 'PCI_LTSSM_L0' undeclared (first use in this function)

Signed-off-by: Roy Zang <tie-fei.zang@freescale.com>
Signed-off-by: York Sun <yorksun@freescale.com>