csky: Add arch_show_interrupts for IPI interrupts

Here is the result:

 cat /proc/interrupts
           CPU0       CPU1       CPU2       CPU3
 15:       1348       1299        952       1076  C-SKY SMP Intc  15  IPI Interrupt
 16:       1203       1825       1598       1307  C-SKY SMP Intc  16  csky_mp_timer
 43:        292          0          0          0  C-SKY SMP Intc  43  ttyS0
 57:        106          0          0          0  C-SKY SMP Intc  57  virtio0
IPI0:         0          0          0          0  Empty interrupts
IPI1:        19         41         45         27  Rescheduling interrupts
IPI2:      1330       1259        908       1050  Function call interrupts
IPI3:         0          0          0          0  Irq work interrupts

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@arndb.de>