clk: meson: check pll rate param table before using it

Make sure the rate param table is available before using it.
Some read-only plls don't provide it, which is ok since the
table is not used by read-only clocks. R/W clocks are supposed
to provide it, but it does not hurt check it.

Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>