microblaze: Add missing chosen node

fdtgrep requires /chosen node to be specified or at least more nodes which
stays in DTS to generate output.
Error message:
./tools/fdtgrep -b u-boot,dm-pre-reloc -RT dts/dt.dtb -n /chosen -O
dtb | ./tools/fdtgrep -r -O dtb - -o spl/u-boot-spl.dtb -P pinctrl-0 -P
pinctrl-names -P clocks -P clock-names -P interrupt-parent
Error: FDT_ERR_BADMAGIC

This patch add empty chosen node to keep fdtgrep happy and pass
compilation for in tree DTS file.

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