mmc: fsl_esdhc: get clock directly from global data

This patch is to get clock directly from global data.

- Remove uclass clk api method. This was what i.MX platforms were
  using, while QorIQ platforms weren't.
- Get clock only from global data, dropping mxc_get_clock().
  QorIQ eSDHC controllers on one silicon use same reference clock.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>