ppc4xx: Add SDRAM detection for PMC440 boards

This patch adds support to detect the amount of DDR2 SDRAM
on PMC440 modules. Detection is done by probing through
a list of available and supported hardware configurations
from 1GByte down to 256MB.

The static TLB entry is replaced by dynamically created entries.

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