microblaze: Add missing quotes around XILINX_MICROBLAZE0_HW_VER

This issue is reported by kconfiglib:
warning: style: quotes recommended around default value for string
symbol XILINX_MICROBLAZE0_HW_VER (defined at
board/xilinx/microblaze-generic/Kconfig:37)

Signed-off-by: Michal Simek <michal.simek@xilinx.com>