powerpc/fsl-booke: fix the case where we are not in the first page

During boot we change the mapping a few times until we have a "defined"
mapping. During this procedure a small 4KiB mapping is created and after
that one a 64MiB. Currently the offset of the 4KiB page in that we run
is zero because the complete startup up code is in first page which
starts at RPN zero.
If the code is recycled and moved to another location then its execution
will fail because the start address in 64 MiB mapping is computed
wrongly. It does not consider the offset to the page from the begin of
the memory.
This patch fixes this. Usually (system boot) r25 is zero so this does
not change anything unless the code is recycled.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>