OMAP2+: powerdomain: add API to get context loss count

Add new powerdomain API

    u32 pwrdm_get_context_loss_count(struct powerdomain *pwrdm)

for checking how many times the powerdomain has lost context.  The
loss count is the sum of the powerdomain off-mode counter, the
logic off counter and the per-bank memory off counter.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
[paul@pwsan.com: removed bogus return value on error; improved kerneldoc;
 tweaked commit message]
Signed-off-by: Paul Walmsley <paul@pwsan.com>