powerpc/8xx: Shorten irq_chip name for the SIU

powerpc/8xx: Shorten irq_chip name for the SIU

show_interrupts() expects the irq_chip name to be max 8 characters
otherwise everything get misaligned

# cat /proc/interrupts
           CPU0
 17:          0   CPM PIC   0 Level     error
 19:          0  MPC8XX SIU  15 Level     tbint
 20:         90   CPM PIC   4 Level     cpm_uart
 38:      29746  MPC8XX SIU   5 Level     fs_enet-mac
 39:          0  MPC8XX SIU   7 Level     fs_enet-mac
 47:        401   CPM PIC   5 Level     fsl_spi
 68:          1  MPC8XX SIU   2 Level     phy_interrupt, phy_interrupt, phy_interrupt
LOC:    7225485   Local timer interrupts for timer event device
LOC:          9   Local timer interrupts for others
SPU:          0   Spurious interrupts
PMI:          0   Performance monitoring interrupts
MCE:          0   Machine check exceptions

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>