lantiq: fix pci issue if mem kernel parameter is used

As specified by e0229a16 if the VR9
based router provides FXS ports and they should enabled then the
following must added to the kernel command line:

  mem=[TOTALMEMSIZE-2M] vpe1_load_addr=ADDRESS vpe1_mem=2M

By adding mem= parameter a pci device stop working correctly. The
pci-lantiq.c module use get_num_physpages() to compute dynamically the
memory amount of the board.

The mem= make the module to compute in the wrong way the BAR11MASK, so
in this situation the mask is misaligned with the dma area that the
hardware expects.

This patch is a port of what legacy ifxmips_pci.c does.

Signed-off-by: Eddi De Pieri <eddi@depieri.net>