mfd: Add ACPI support

If an MFD device is backed by ACPI namespace, we should allow subdevice
drivers to access their corresponding ACPI companion devices through normal
means (e.g using ACPI_COMPANION()).

This patch adds such support to the MFD core. If the MFD parent device
does not specify any ACPI _HID/_CID for the child device, the child
device will share the parent ACPI companion device. Otherwise the child
device will be assigned with the corresponding ACPI companion, if found
in the namespace below the parent.

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>