enable grub payload on libremrc w541/t440p
    
    the grub payload was previously disabled, because the libre
    mrc code sets up xhci rather than ehci, and grub did not have
    xhci support (not natively).
    
    libreboot now has xhci support in the grub payload, so enable
    grub on these configurations.
    
    Signed-off-by: Leah Rowe <leah@libreboot.org>