clk: Add missing documentation of devm_clk_bulk_get_optional() argument

Fix an incomplete devm_clk_bulk_get_optional() function documentation
by adding description of the num_clks argument as in other *clk_bulk*
functions.

Fixes: 9bd5ef0b ("clk: Add devm_clk_bulk_get_optional() function")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>