clock management changes for i.MX

Another simple series related to clock management, this time only for
imx.
Merge branch 'imx/clk' into next/clk

* imx/clk:
  ARM: mxs: select HAVE_CLK_PREPARE for clock
  clk: add config option HAVE_CLK_PREPARE into Kconfig
  ASoC: mxs-saif: convert to clk_prepare/clk_unprepare
  video: mxsfb: convert to clk_prepare/clk_unprepare
  serial: mxs-auart: convert to clk_prepare/clk_unprepare
  net: flexcan: convert to clk_prepare/clk_unprepare
  mtd: gpmi-lib: convert to clk_prepare/clk_unprepare
  mmc: mxs-mmc: convert to clk_prepare/clk_unprepare
  dma: mxs-dma: convert to clk_prepare/clk_unprepare
  net: fec: add clk_prepare/clk_unprepare
  ARM: mxs: convert platform code to clk_prepare/clk_unprepare
  clk: add helper functions clk_prepare_enable and clk_disable_unprepare