ppc4xx: Allow overwriting pci target registers for all 4xx boards

This patch adds the CONFIG_PCI_4xx_PTM_OVERWRITE option and replaces
the ugly 'if defined(BOARD1) || ... || defined(BOARDn)' construct
in 4xx pci code.

When CONFIG_PCI_4xx_PTM_OVERWRITE is defined the default ptm register
setup can be overwritten through environment variables ptm1la, ptm1ms,
ptm2la and ptm2ms to do application specific pci target BAR configuration.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd.eu>
Signed-off-by: Stefan Roese <sr@denx.de>