External m.d. interface points:

To config, code should configure 'gt' with a struct gtbus_attach_args
filled in.  The code should have already set up the bus_space and
bus_dma tags that are passed in.

m.d. pci_chipset_tag_t should contain at least a copy of the
gtpci_chipset_tag_t data structure.

m.d. code must provide the following functions:

pci_intr_map
pci_intr_string
pci_intr_establish,
pci_intr_disestablish

void *gtmpp_intr_establish(struct gt_softc *gt, int mpp_pin, int ipl,
		     int (*handler)(void *), void *arg);
	Add an interrupt attached to Discovery MPP pin <mpp_pin> at
	the given ipl with the given handler.

void gtmpp_intr_disestablish(struct gt_softc *gt, void *cookie);
	Remove an interrupt attached to Discovery MPP pin.

int gtget_macaddr(struct gt_softc *gt, int function, char *enaddr);
	Get MAC for Discovery ethernet <function>

void intr_enable(void);
	Enable all interrupts (disregarding spls)

void intr_disable(void);
	Disable all interrupts (disregarding spls)