Improve additional interrupt ACK for Broadcom XLP

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.