Handling some interrupts in XLP (like PCIe and SATA) involves writing to
vendor specific registers as part of interrupt acknowledgement.
This was earlier done with xlp_establish_intr(), but a better solution
is to provide a function xlp_set_bus_ack() that can be used with
cpu_establish_hardintr(). This will allow platform initialization code to
setup these ACKs without changing the standrard drivers.
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