powerpc: Add HAVE_IRQ_TIME_ACCOUNTING

Allow us to enable IRQ_TIME_ACCOUNTING. Even though we currently
use VIRT_CPU_ACCOUNTING_NATIVE, that option is quite heavy
weight and IRQ_TIME_ACCOUNTING might be better in some cases.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>