powerpc/corenet: explicitly disable the SDHC controller on kmcoge4

[ Upstream commit a674c7d470bb47e82f4eb1fa944eadeac2f6bbaf ]

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>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>