[RFC] Add PowerPC MPC5125 support

[Juergen Borleis]

This is a try to add the MPC5125 SoC to Barebox. This kind of SoC shares some
embedded devices with the MPC5200 and the MPC5121/MPC5123 (at least it uses
the same names for them) but differs in many annoying details. So this
adaption can only be used on a MPC5125 and is completely untested on MPC5121
and MPC5123.

Patch "arch/MPC5xxx: fix linker script for MPC5200" tries to repair the linker
script for the MPC5200 because the resulting binary image is broken without
this change. But I do not really know why it is broken and how to repair
it correctly. Comments?

At least the existing PSC code for the serial UART can be shared between both
SoCs, even if their internals differ. With some re-factoring it is possible
to use the main code on both SoCs.

The same might be possible with the FEC driver, but needs more effort, because
the DMA work differently on both SoCs. I'm not sure if its worth the effort,
so I just add a new FEC driver for the MPC5125 with this series.

Comments are welcome.

Juergen