ppc/pnv: add a ISA bus

ppc/pnv: add a ISA bus

As Qemu only supports a single instance of the ISA bus, we use the LPC
controller of chip 0 to create one and plug in a couple of useful
devices, like an UART and RTC. An IPMI BT device, which is also an ISA
device, can be defined on the command line to connect an external BMC.
That is for later.

The PowerNV machine now has a console. Skiboot should load a kernel
and jump into it but execution will stop quite early because we lack a
model for the native XICS controller for the moment :

    [    0.000000] NR_IRQS:512 nr_irqs:512 16
    [    0.000000] XICS: Cannot find a Presentation Controller !
    [    0.000000] ------------[ cut here ]------------
    [    0.000000] WARNING: at arch/powerpc/platforms/powernv/setup.c:81
    ...
    [    0.000000] NIP [c00000000079d65c] pnv_init_IRQ+0x30/0x44

You can still do a few things under xmon.

Based on previous work from :
      Benjamin Herrenschmidt <benh@kernel.crashing.org>

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
[dwg: Trivial fix for a change in the serial_hds_isa_init() interface]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>