microblaze: Enabling CONFIG_BRIDGE in mmu_defconfig

This patch enables CONFIG_BRIDGE=m by default to be aligned
with Xilinx defaults.

Signed-off-by: Vineeth Chowdary Karumanchi <vineethchowz.chowdary@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>