clk: fix mux clock documentation

The mux documentation mentions the non-existing parameter width instead
of mask, so just sed this.

The table field is missing in the documentation of clk_mux.
Add a small blurb explaining what it is

Fixes: 9d9f78ed ("clk: basic clock hardware types")
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>