Hi Daniele,
Looks like issue in MK22FX512xxx12 / MK22FX512Axxx12 SDK packages. I forwarded it to SDK development team. As a workaround, you can download packages for MK21FX512xxx12 / MK21FX512Axxx12 device and take the missing fsl_sdhc.c/.h files from there.
Regards,
Lukas