ARM: OMAP: Wakeupgen: Add context save/restore for AM43XX

AM43XX has the same wakeupgen IP as OMAP4/5. The only
notable difference is the presence of 7 register banks
and lack of SAR area which has been used in OMAP4/5 for
saving and restoring the context around low power states.

In case of AM43XX the context is saved and restored by
the kernel. Introduce wakeupgen_ops so that context save
and restore can be set on a per-SoC basis during init.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>