microblaze: Clear i/dcache for sw breakpoints

There is necessary to flush dcache and invalidate
icache for address where breakpoint (brki r16, 0x18)
was. The reason is that for some cases icache line
still keeps brki instruction and it is not updated
by origin instruction maintains by gdbserver.

Signed-off-by: Michal Simek <monstr@monstr.eu>