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>
diff --git a/arch/microblaze/Makefile b/arch/microblaze/Makefile
index 538afce..72f6e85 100644
--- a/arch/microblaze/Makefile
+++ b/arch/microblaze/Makefile
@@ -94,3 +94,5 @@
   echo '  name of a dts file from the arch/microblaze/boot/dts/ directory'
   echo '  (minus the .dts extension).'
 endef
+
+MRPROPER_FILES += $(boot)/simpleImage.*