Support Driver Model for fsl esdhc driver.

1. Introduce a new structure struct fsl_esdhc_priv
2. Refactor fsl_esdhc_initialize which is originally used by board code.
   - Introduce fsl_esdhc_init to be common usage for DM and non-DM
   - Introduce fsl_esdhc_cfg_to_priv to build the bridge for non-DM part.
   - The original API for board code is still there, but we use
     'fsl_esdhc_cfg_to_priv' and 'fsl_esdhc_init' to serve it.
3. All the functions are changed to use 'struct fsl_esdhc_priv', except
   fsl_esdhc_initialize.
4. Since clk driver is not implemented, use mxc_get_clock to geth
   the clk and fill 'priv->sdhc_clk'.

Has been tested on i.MX6UL 14X14 EVK board:
"
=>dm tree
....
 simple_bus  [ + ]    |   `-- aips-bus@02100000
  mmc        [ + ]    |       |-- usdhc@02190000
  mmc        [ + ]    |       |-- usdhc@02194000
....
=> mmc list
FSL_SDHC: 0 (SD)
FSL_SDHC: 1 (SD)
"

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Yangbo Lu <yangbo.lu@nxp.com>
Cc: Hector Palacios <hector.palacios@digi.com>
Cc: Eric Nelson <eric@nelint.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@nxp.com>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Simon Glass <sjg@chromium.org>
Tested-By: Eric Nelson <eric@nelint.com>
Reviewed-by: York Sun <york.sun@nxp.com>