85xx: Fix memory test range on MPC8536DS

With current values of CONFIG_SYS_MEMTEST_START and CONFIG_SYS_MEMTEST_END
memory test hangs if run without arguments. Set them to sane values, so
that all available 512MB of RAM excluding exception vectors at the bottom
and u-boot code and stack at the top can be tested.

Signed-off-by: Felix Radensky <felix@embedded-sol.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>