microblaze: remove unecessary of_platform_bus_probe call

The call to of_platform_bus_probe has no effect because the DT core
already probes default buses like "simple-bus" before this call.
Michal Simek said 'xlnx,compound' hasn't been used in a long time, so
that match entry isn't needed.

Cc: Michal Simek <monstr@monstr.eu>
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>