clk: max77686: Register OF clock provider

If max77686 chip is instantiated from device tree, it is desirable to
have an OF clock provider to allow device tree based look-up of clocks.
This patch adds OF clock provider registration to the clk-max77686
driver.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>