ARM: OMAP4+: CPUidle: Deprecate use of omap4_mpuss_read_prev_context_state()

Current OMAP4 CPUIdle driver is using omap4_mpuss_read_prev_context_state()
to check whether the MPU cluster lost context or not before calling
cpu_cluster_pm_exit().  This was initially done an optimization for
corner cases, where if the cluster low power entry fails for some
reason, the cluster context restore gets skipped.  However, since
reading the previous context is expensive (involving slow accesses to
the PRCM), it's better to avoid it and simply check the target cluster
state instead.

Moving forward, OMAP CPUidle drivers needs to be moved to drivers/idle/*
once the PRM/CM code gets moved to drivers. This patch also reduces one
dependency with platform code for idle driver movement.

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