clk: imx: Fix and update kerneldoc

Fix and add missing kerneldoc to fix compile warnings like:

  drivers/clk/imx/clk-pfd.c:27: warning: Function parameter or member 'hw' not described in 'clk_pfd'
  drivers/clk/imx/clk-pllv3.c:53: warning: Function parameter or member 'ref_clock' not described in 'clk_pllv3'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>