clk: ti: dra7x: prevent non-existing clkctrl clocks from registering

Certain clkctrl clocks (like the USB_OTG_SS4) do not exist on some
variants of the dra7x SoC. Append a flag for these clocks and skip
the registration in cases where the clocks do not exist.

Reported-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>