hw/alpha: Provide a PCI-ISA bridge device node

- Move initialization of the ISA bus from typhoon_init() to clipper_init();
  this apsect of device topology is really associated with the individual
  model, not the core logic chipset.  typhoon_init() now returns the IRQ
  to use for the output of the ISA PIC.

- In clipper_init(), instantiate an i82378 instance, and connect its
  PIC output to the ISA IRQ input provided by typhoon_init().  Remove
  the explicit instantiations of i8254 and i82374, as these devices
  are subsumed by the i82378.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Jason Thorpe <thorpej@me.com>
Message-Id: <20210616141538.25436-1-thorpej@me.com>
[rth: Remove direct dependencies on i82374, i8254, i8259.]
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>