clk: si5351: expand compatible strings in documentation

checkpatch.pl doesn't know how to expand "silabs,si5351{a,a-msop,b,c}"
and so generates warnings about si5351-compatible devices appearing to
be un-documented. Resolve this by documenting the compatible options
supported by the clk-si5351 driver individually.

Signed-off-by: Brandon Streiff <brandon.streiff@ni.com>
Acked-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Rob Herring <robh@kernel.org>