AP_Mount: set clock on SIYI #32682

Job	Run time
build (base)	5m 22s
build-gcc-heli	5m 9s
build (clang)	6m 3s
autotest-heli (sitltest-heli)	11m 1s
autotest (sitltest-copter-tests1a)	11m 33s
autotest (sitltest-copter-tests1b)	13m 39s
autotest (sitltest-copter-tests1c)	12m 31s
autotest (sitltest-copter-tests1d)	11m 40s
autotest (sitltest-copter-tests1e)	13m 18s
autotest (sitltest-copter-tests2a)	13m 20s
autotest (sitltest-copter-tests2b)	23m 59s
	2h 7m 35s

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