openrisc: fix relocation code

The relocation code can now relocate from anywhere to
the RAM.

The old code assumed that the binary was copied to the RAM
by some PBL and then it just relocated the .text section
from the loaded address to the linked address.

Now, it first checks if vectors are somewhere else than the
linked address. If yes, there are copied to address 0 (or
to the exception vector base address if register EVBAR is
present).

Then, the .text section is relocated from its current location
to the RAM.

Signed-off-by: Franck Jullien <franck.jullien@gmail.com>