[POWERPC] fsl_soc: Fix get_immrbase() to use ranges, rather than reg.

Don't depend on the reg property as a way to determine the base
of the immr space.  The reg property might be defined differently for
different SoC families.

Signed-off-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>