clk: imx: clk-gpr-mux: Simplify .determine_rate()

imx_clk_gpr_mux_determine_rate() is the same as __clk_mux_determine_rate(),
so use the latter to save some LoC.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/ac8bd50c41b84f244bb0ec94e8aed25c513c9037.1688760152.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Abel Vesa <abel.vesa@linaro.org>