ARM: OMAP4: PM: Avoid expensive cpu_suspend() path for all CPU power states except off

Current CPU PM code code make use of common cpu_suspend() path for all the
CPU power states which is not optimal. In fact cpu_suspend() path is needed
only when we put CPU power domain to off state where the CPU context is lost.

Update the code accordingly so that the expensive cpu_suspend() path
can be avoided for the shallow CPU power states like CPU PD INA/CSWR.

The patch has been tested on OMAP4430 and OMAP5430(with few out of tree patches)
devices for suspend and CPUidle.

Cc: Kevin Hilman <khilman@deeprootsystems.com>

Reported-by: Richard Woodruff <r-woodruff2@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>