omap3: cpuidle: Add description field to each C-state.

Add a description field to each idle C-state. This helps to give
better data with PowerTop and one don't have to refer to the code
to link what Cx means from system point of view while analysing
PowerTop data.

No functional change.

Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Signed-off-by: Kevin Hilman <khilman@ti.com>