docs: pmu: fix binding example

The first PMU binding example does not terminate properties with a ;,
which is invalid. Noticed while converting the binding to yaml.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Anup Patel <anup@brainfault.org>