clk: Add clk_bulk_get_optional() function

clk_bulk_get_optional() allows to get a group of clocks where one
or more is optional.  For a not available clock, e.g. not specifed
in the clock consumer node in DT, its respective struct clk pointer
will be NULL.  This allows for operating on a group of returned
clocks (struct clk_bulk_data array) with existing clk_bulk* APIs.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>