Use devm_clk_get_optional() instead of devm_clk_get() and explicit
optional clock handling.
As clk_prepare_enable() and clk_disable_unprepare() handle optional
clocks fine, the cc_clk_on() and cc_clk_off() wrappers can be removed.

While at it, use the new "%pe" format specifier to print error codes.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>