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>