ppc4xx: Make 440SPe PCIe code more generic to use on different 4xx PPCs (3)

(3) This patch introduces macros like SDRN_PESDR_DLPSET(port) to access
    the SDR registers of the PCIe ports. This makes the overall design
    clearer, since it removed a lot of switch statements which are not
    needed anymore.

    Also, the functions ppc4xx_init_pcie_rootport() and
    ppc4xx_init_pcie_entport() are merged into a single function
    ppc4xx_init_pcie_port(), since most of the code was duplicated.
    This makes maintainance and porting to other 4xx platforms
    easier.

Signed-off-by: Stefan Roese <sr@denx.de>