Blackfin arch: Fix bug - KGDB single step into the middle of a 4 bytes instruction...

Run IFLUSH twice to avoid loading wrong instruction
after invalidating icache and following sequence is met.

1) The one instruction address is cached in the icache.
2) This instruction in SDRAM is changed.
3) IFLASH[P0] is executed only once in lackfin_icache_flush_range().
4) This instruction is executed again, but not the changed new one.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>