powerpc: Rename PMU interrupts from CNT to PMI

Back in commit 89713ed1 "Add timer, performance monitor and machine check
counts to /proc/interrupts" we added a count of PMU interrupts to the
output of /proc/interrupts.

At the time we named them "CNT" to match x86.

However in commit 89ccf465

 "Rename 'performance counter interrupt'", the
x86 guys renamed theirs from "CNT" to "PMI".

Arguably changing the name could break someone's script, but I think the
chance of that is minimal, and it's preferable to have a name that 1) is
somewhat meaningful, and 2) matches x86.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>