arm/ti: correct softc member name in ti_mux_clock and ti_gate_clock

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Summary

18250ec6c089 ("Replace calls to bus_generic_attach with
bus_attach_children") changed three calls in each of ti_mux_clock.c and
ti_gate_clock.c to bus_attach_children(sc->dev), but these softcs name
the member sc_dev, so neither file compiles. Nothing caught it because
no kernel config in the tree builds these files since AM335x was
removed from armv7 GENERIC in 3416e102c4e9.

Use sc->sc_dev, as the neighbouring lines already do.

Test Plan

As with D59118, compile coverage requires an AM335x kernel configuration
(see D46703).

Verified by building KERNCONF=AM335X for TARGET=arm TARGET_ARCH=armv7 on
stable/15: the files compile, and the resulting kernel boots to multi-user
on a BeagleBone Black Rev B3 with the TI clock tree registering (mux,
gate, divider and DPLL nodes all attach).