clk: fixed: add missing kerneldoc

Add missing kerneldoc to fix compile warnings like:

  drivers/clk/clk-fixed-factor.c:211: warning: Function parameter or member 'node' not described in 'of_fixed_factor_clk_setup'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200916161740.14173-2-krzk@kernel.org


Signed-off-by: Stephen Boyd <sboyd@kernel.org>