soc: ti: pm33xx: Enable basic PM runtime support for genpd

To prepare for moving to use genpd, let's enable basic PM
runtime support.

Cc: Dave Gerlach <d-gerlach@ti.com>
Cc: Santosh Shilimkar <ssantosh@kernel.org>
Cc: Suman Anna <s-anna@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>