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>

arch/powerpc/include/asm/arch-mpc83xx/clock.h

-/*
- * (C) Copyright 2018
- * Mario Six, Guntermann & Drunck GmbH, mario.six@gdsys.cc
- *
- * SPDX-License-Identifier:	GPL-2.0+
- */
-
-#ifndef __ASM_POWERPC_CLOCK_H
-#define __ASM_POWERPC_CLOCK_H
-
-/* Make fsl_esdhc driver happy */
-enum mxc_clock {
-	MXC_ESDHC_CLK,
-};
-
-DECLARE_GLOBAL_DATA_PTR;
-
-uint mxc_get_clock(int clk)
-{
-	return gd->arch.sdhc_clk;
-}
-#endif /* __ASM_POWERPC_CLOCK_H */