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

[ Upstream commit 74033131 ]

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>
Stable-dep-of: 8f3c307b ("soc: ti: pm33xx: Fix refcount leak in am33xx_pm_probe")
Signed-off-by: Sasha Levin <sashal@kernel.org>