powerpc/corenet: explicitly disable the SDHC controller on kmcoge4

It is not implemented on the kmcoge4 hardware and if not disabled it
leads to error messages with the corenet32_smp_defconfig.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Scott Wood <oss@buserror.net>