qcom_gcc: migrate to use sleep_clk directly

gcc_sleep_clk_src does not exist, just use sleep_clk directly.
I believe this is what feeds them anyway!

This node isn't in the Linux clock driver (drivers/clk/qcom) ipq401x
clock tree, but there are sleep_clk_src nodes in the tree in
ipq50xx, ipq54xx, ipq60xx, ipq8xxxx, ipq9xxxx, msm8xxx, etc.

Differential Revision: https://reviews.freebsd.org/D49686