clk: abstract parent cache

Abstract access to the clock parent cache by defining
clk_get_parent_by_index(clk, index). This allows access to parent
clocks from clock drivers.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Mike Turquette <mturquette@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org
Signed-off-by: Mike Turquette <mturquette@linaro.org>