powerpc/fsl-booke: Fix address issue when using relocatable kernels

When booting a relocatable kernel it needs to jump to the correct
start address, which for BookE parts is usually unchanged
regardless of the physical memory offset.

Recent changes cause problems with how we calculate the start
address, it was always adding the RMO into the start address
which is incorrect. This patch only adds in the RMO offset
if we are in the kexec code path, as it needs the RMO to work
correctly.

Instead of adding the RMO offset in in the common code path, we
can just set r6 to the RMO offset in the kexec code path instead
of to zero, and finally perform the masking in the common code
path

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>