powerpc/irq: use seq_put_decimal_ull_width() for decimal values

On a system with n CPUs and m interrupts, there will be n*m decimal
values yielded via seq_printf(.."%10u "..) which is less efficient
than seq_put_decimal_ull_width(), stress reading /proc/interrupts
indicates ~30% performance improvement with this patch.

Signed-off-by: David Wang <00107082@163.com>
[mpe: Flesh out change log based on original submission]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/all/20241103080552.4787-1-00107082@163.com
Link: https://patch.msgid.link/20241108162327.9887-1-00107082@163.com