xilinx: move microblaze-generic .dts to standard location

Aside from microblaze, all other SoCs/boards/vendors store their DT files
in board/$vendor/dts/$soc-$board.dts. Move microblaze-generic.dts to this
location for consistency.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Michal Simek <monstr@monstr.eu>