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

include/configs/MPC8536DS.h

@@ -86,8 +86,8 @@ extern unsigned long get_board_ddr_clk(unsigned long dummy);
 #define CONFIG_SYS_NUM_ADDR_MAP		16	/* number of TLB1 entries */
 #endif
 
-#define CONFIG_SYS_MEMTEST_START	0x00000000	/* memtest works on */
-#define CONFIG_SYS_MEMTEST_END		0x7fffffff
+#define CONFIG_SYS_MEMTEST_START	0x00010000	/* skip exception vectors */
+#define CONFIG_SYS_MEMTEST_END		0x1f000000      /* skip u-boot at top of RAM  */
 #define CONFIG_PANIC_HANG	/* do not reset board on panic */
 
 /*