[doxygen] add doxygen comment for clk.c and clk.h #5120

Job	Run time
RT-Thread Online Packages (STM32F407 RT-Spark)	6m 36s
RTduino/Arduino Libraries (STM32F412 Nucleo)	15m 20s
RTduino/Arduino Libraries (Raspberry Pico)	12m 2s
others_at32_hc32_ht32	7m 50s
stm32l4_f0_f1	13m 19s
stm32_f2_f4	11m 1s
stm32_f7_g0_h7_mp15_u5_h5_wb5	5m 38s
nxp_renesas	6m 10s
nuvoton	5m 39s
gd32_n32_apm32	5m 39s
Infineon_TI_microchip	7m 51s
mips	3m 6s
aarch64	5m 17s
riscv-none	5m 18s
riscv64-unknown	4m 43s
hpmicro	2m 41s
llvm-arm	5m 35s
simulator	3m 54s
ESP32C3	24m 46s
i386-unknown	3m 9s
nordic(yml)	12m 41s
	2h 48m 15s

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