MicroBlaze: Update software breakpoint machine code as per MicroBlaze ISA

This patch updates breakpoint instruction machine code to be inline
with the specification in MicroBlaze ISA [1]

* gdb/microblaze-tdep.h : Update MICROBLAZE_BREAKPOINT

[1] https://docs.amd.com/r/en-US/ug984-vivado-microblaze-ref/brki

Signed-off-by: David Holsgrove <david.holsgrove@petalogix.com>
Signed-off-by: Nathan Rossi <nathan.rossi@petalogix.com>
Signed-off-by: Mahesh Bodapati <mbodapat@xilinx.com>
Signed-off-by: Gopi Kumar Bulusu <gopi@sankhya.com>