clk: s2mps11: Fix clk_ops

s2mps11 clocks had registered callbacks for prepare ,unprepare and is_enabled.
During disabling unused clocks the lack of is_prepared caused that unused
s2mps11 clocks were not unprepared and stayed active.

Regmap_read is cached so it can be called in is_prepare callback
to achieve this information. Enabled field was removed from struct s2mps11_clk.

Signed-off-by: Karol Wrona <k.wrona@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>