microblaze: Support stack protection feature

Ensure that stack didn't rewrite important part
of u-boot.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/cpu/start.S

@@ -23,11 +23,15 @@ _start:
 
 	mts	rmsr, r0	/* disable cache */
 
+	addi	r8, r0, __end
+	mts	rslr, r8
 #if defined(CONFIG_SPL_BUILD)
 	addi	r1, r0, CONFIG_SPL_STACK_ADDR
+	mts	rshr, r1
 	addi	r1, r1, -4	/* Decrement SP to top of memory */
 #else
 	addi	r1, r0, CONFIG_SYS_INIT_SP_OFFSET
+	mts	rshr, r1
 	addi	r1, r1, -4	/* Decrement SP to top of memory */
 
 	/* Find-out if u-boot is running on BIG/LITTLE endian platform