clk: Add devm_clk_bulk_get_optional() function

Add managed version of the clk_bulk_get_optional() helper function.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
[sboyd@kernel.org: Mark __devm_clk_bulk_get() static]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>