platform: generic: Add PMU support

Add PMU support for generic platform. Generic platform solely relies on
the device tree to parse all pmu related information. If any event is
not described in device tree, generic platform will not support it.

Reviewed-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>