clk: imx: get stdout clk count from device tree

Currently the clk_count is specified by API users, but this
parameter is wrongly used, for example, i.MX8M clk driver use 4,
however the uart device tree node only use 2 clock entries. So
let using of_clk_get_parent_count to get the exact clock count.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Link: https://lore.kernel.org/r/20230104110032.1220721-3-peng.fan@oss.nxp.com