ARM: OMAP4+: CPUidle: Consolidate idle driver for OMAP5 support

The OMAP5 idle driver can re-use most of OMAP4 CPUidle driver
implementation. Also the next derivative SOCs are going to re-use
the MPUSS so, same driver with minor updates can be re-used.

Prepare the code so that its easier to add CPUidle support for
OMAP5 devices.

Acked-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Kevin Hilman <khilman@linaro.org>