microblaze: Wire up defconfig to mmu_defconfig

Without this, "make defconfig" fails with

*** Can't find default configuration "arch/microblaze/defconfig"!

All other architectures either set KBUILD_DEFCONFIG, or provide
arch/*/defconfig.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>