microblaze: Fix Makefile to delete build generated files

'make clean' does not to delete the following build generated file:
	arch/microblaze/boot/linux.bin.ub

'make mrproper' does not to delete the following build generated files:
	arch/microblaze/boot/simpleImage.*

Fix the Makefile to delete these build generated files.

See [1] for a discussion on why simpleImage.* files are deleted with 'make
mrproper' and not with 'make clean'.
	[1] http://lkml.org/lkml/2010/3/12/96

Signed-off-by: Arun Bhanu <arun@bhanu.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/Makefile

@@ -94,3 +94,5 @@ define archhelp
   echo '  name of a dts file from the arch/microblaze/boot/dts/ directory'
   echo '  (minus the .dts extension).'
 endef
+
+MRPROPER_FILES += $(boot)/simpleImage.*

arch/microblaze/boot/Makefile

@@ -64,4 +64,4 @@ $(obj)/%.dtb: $(dtstree)/%.dts FORCE
 
 clean-kernel += linux.bin linux.bin.gz simpleImage.*
 
-clean-files += *.dtb simpleImage.*.unstrip
+clean-files += *.dtb simpleImage.*.unstrip linux.bin.ub