clk: rockchip: use match_string() helper

match_string() returns the index of an array for a matching string,
which can be used intead of open coded variant.

Cc: Michael Turquette <mturquette@baylibre.com>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Cc: linux-clk@vger.kernel.org
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-rockchip@lists.infradead.org
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>