Add PPC4xx-HCU4 and HCU5 boards: Infrastructure

This series of patches adds support for 2 boards from Netstal Maschinen.

The HCU4 has a PPC405Gpr and
the HCU5 has a PPC440EPX.

The HCU4 has a somehow complicated flash setup, as the booteprom is
only 8 bits and the CFI 16 bits wide, which makes it impossible to use a more
elegant solution.

The HCU5 has only a booteprom as the whole code will be downloaded from a
different board which has HD, CD-ROM, etc and where all code is stored.

This is my third try. I incorporated all suggestions made by Wolfgang and Stefan.
Thanks them a lot.

Signed-off-by: Niklaus Giger <niklaus.giger@netstal.com>