mpc83xx: remove unused clock.h

The clock.h was to define mxc_get_clock() providing clock value
to fsl_esdhc driver. Since fsl_esdhc driver is using global data
gd->arch.sdhc_clk directly now, we can remove this file.

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