powerpc/time: Enable sched clock for irqtime

When CONFIG_IRQ_TIME_ACCOUNTING and CONFIG_VIRT_CPU_ACCOUNTING_GEN, powerpc
does not enable "sched_clock_irqtime" and can not utilize irq time
accounting.

Like x86, powerpc does not use the sched_clock_register() interface. So it
needs an dedicated call to enable_sched_clock_irqtime() to enable irq time
accounting.

Fixes: 518470fe ("powerpc: Add HAVE_IRQ_TIME_ACCOUNTING")
Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
[mpe: Add fixes tag]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1603349479-26185-1-git-send-email-kernelfans@gmail.com