ARM: OMAP2+: clock/interface: add a clk_features definition for idlest value

Helps to get rid of some runtime cpu_is_x checks. This also allows eventual
migration of the code under clock driver.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Reviewed-by: Mike Turquette <mturquette@linaro.org>
Signed-off-by: Paul Walmsley <paul@pwsan.com>