Revert "[media] mfd: Add header files and Kbuild plumbing for SI476x MFD core"

As requested by Samuel Ortiz <sameo@linux.intel.com>, revert
this patch.

This reverts commit 3f8ec5df.

Conflicts:
	drivers/mfd/Kconfig

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>