powerpc/85xx: add CONFIG_SYS_TEXT_BASE_SPL for 85xx nand spl build

Introduce a SPL specific CONFIG_SYS_TEXT_BASE_SPL define to be used by
the linker.  This has similiar semantics to CONFIG_SYS_TEXT_BASE however
since SPL is a unqiue image we introduce a new variable to control its
text base address.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>