staging: fsl-mc: Do not allow building as a module

This driver uses functionality (MSI IRQ domain) whose symbols aren't
exported, and hence the modular build fails. While arguably there might
be reasons to make these symbols available to modules, that change would
be fairly involved and the set of exported functions should be carefully
auditioned. Fix the build failure for now by marking the driver boolean.

Cc: J. German Rivera <German.Rivera@freescale.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>