[Patch, microblaze]: Add supervisory call support

A function can be executed in supervisory mode when a function
is declared with svc_handler attribute.

If a function with svc_handler is called, the processor switches
to supervisory mode and executes. After executing the function
it returns to normal mode.

Same feature can also be invoked with svc_table_handler attribute
but this attribute will accept a service ID, to be used for
calling the function.

Signed-off-by:nagaraju <nmekala@xilix.com>
Signed-off-by: David Holsgrove <david.holsgrove@xilinx.com>