arm: drop eSDHC clock getting in mxc_get_clock() for layerscape

Although layerscape platforms reuse mxc_get_clock() of i.MX platforms,
eSDHC clock getting do not have to use it. It uses global data
gd->arch.sdhc_clk directly in fsl_esdhc driver. Even there are more
than one eSDHC controllers on SoC, they use same reference clock.

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