Adding a attach callback function pointer in case a platform needs to execute anything before the standard initialization of the sdhci mem controller. BUG=b:254092907 BRANCH=None TEST=emerge-herobrine coreboot Change-Id: I0f37ec09d083922cad5ecd3c47b184cf3311fe2d Signed-off-by: Shelley Chen <shchen@google.com> Reviewed-on: https://review.coreboot.org/c/coreboot/+/72633 Tested-by: build bot (Jenkins) <no-reply@coreboot.org> Reviewed-by: Douglas Anderson <dianders@chromium.org>