cpufreq: add driver flag for sleepable transitions

The CPUfreq core does not differentiate between ->target() callbacks
that may sleep/block or ones that are fast and return immediately. To
date this has not mattered much since the typical CPUfreq governor calls
the ->target() callback from a workqueue context.

When calling the ->target() callback from a different context, such as a
scheduler load balance operation (see Morten's "energy aware scheduler"
RFC), this detail matters.

This patch introduces a new CPUfreq driver flag for slow ->target()
callbacks that might sleep or block. Drivers may set this flag when
registering in the same way that they might set other flags such as
CPUFREQ_ASYNC_NOTIFICATION or CPUFREQ_NEED_INITIAL_FREQ_CHECK.

Signed-off-by: Mike Turquette <mturquette@linaro.org>