staging: fsl-mc: TODO updates

remove 3 of the remaining TODO items:

   -multiple root fsl-mc buses-- done in patch series starting with
    commit 14f92805 ("staging: fsl-mc: abstract test for existence
    of fsl-mc bus")

   -interrupt support-- done in patch series starting with
    commit 9b1b282c ("irqdomain: Added domain bus token
    DOMAIN_BUS_FSL_MC_MSI")

   -MC command serialization-- done in commit 63f2be5c ("staging:
    fsl-mc: Added serialization to mc_send_command()")

Signed-off-by: Stuart Yoder <stuart.yoder@nxp.com>
Acked-by: German Rivera <german.rivera@nxp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>