clk: si5351: Mention clock-names in the binding documentation

Since the introduction of clk-si5351 the way we should deal with DT provided
clocks has changed from indexed to named clock phandles. Amend the binding
documentation to reflect named clock phandles by clock-names property.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Michael Turquette <mturquette@linaro.org>