omap4: suspend: Add basic system suspend support

This patch adds support for basic suspend doing a CPUx wfi
for OMAP4. All powerdomains are for now are kept programmed
in ON state.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>