m68k: add global variable sdhc_per_clk for m68k

The FSL eSDHC controller supports two reference clocks. They are
platform clock and periperhal clock. The global variable sdhc_clk
has already been used for platform clock.
ColdFire also uses eSHDC controller, as in arm and powerpc,
so adding sdhc_per_clk to arch_global_data.

Signed-off-by: Angelo Durgehello <angelo@kernel-space.org>