xilinx: removed microbalze_pic_init from xilinx.h

xilinx: removed microbalze_pic_init from xilinx.h

This is a microblaze target specific function that belongs outside
of xilinx.h (which is a collection of target independent device model
instantiator functions)

Signed-off-by: Peter A. G. Crosthwaite <peter.crosthwaite@petalogix.com>
Signed-off-by: Edgar E. Iglesias <edgar.iglesias@gmail.com>