[Blackfin] arch: remove useless IRQ_SW_INT defines

IRQ_SW_INT1 and IRQ_SW_INT2 obsolete:
Remove useless defines
Fix SYS_IRQS
Keep numbering scheme, so we don't break existing configurations.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <cooloney@kernel.org>