drivers: mfd: fix dependencies for MFD_RTSX_PCI

We need to include MFD_CORE if this option is enabled, otherwise we get
build errors.

Cc: Wei WANG <wei_wang@realsil.com.cn>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Borislav Petkov <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>