powerpc: Convert power off logic to pm_power_off

The generic Linux framework to power off the machine is a function pointer
called pm_power_off. The trick about this pointer is that device drivers can
potentially implement it rather than board files.

Today on powerpc we set pm_power_off to invoke our generic full machine power
off logic which then calls ppc_md.power_off to invoke machine specific power
off.

However, when we want to add a power off GPIO via the "gpio-poweroff" driver,
this card house falls apart. That driver only registers itself if pm_power_off
is NULL to ensure it doesn't override board specific logic. However, since we
always set pm_power_off to the generic power off logic (which will just not
power off the machine if no ppc_md.power_off call is implemented), we can't
implement power off via the generic GPIO power off driver.

To fix this up, let's get rid of the ppc_md.power_off logic and just always use
pm_power_off as was intended. Then individual drivers such as the GPIO power off
driver can implement power off logic via that function pointer.

With this patch set applied and a few patches on top of QEMU that implement a
power off GPIO on the virt e500 machine, I can successfully turn off my virtual
machine after halt.

Signed-off-by: Alexander Graf <agraf@suse.de>
[mpe: Squash into one patch and update changelog based on cover letter]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/setup.c

@@ -632,6 +632,8 @@ static int __init pmac_probe(void)
 	smu_cmdbuf_abs = memblock_alloc_base(4096, 4096, 0x80000000UL);
 #endif /* CONFIG_PMAC_SMU */
 
+	pm_power_off = pmac_power_off;
+
 	return 1;
 }
 
@@ -663,7 +665,6 @@ define_machine(powermac) {
 	.get_irq		= NULL,	/* changed later */
 	.pci_irq_fixup		= pmac_pci_irq_fixup,
 	.restart		= pmac_restart,
-	.power_off		= pmac_power_off,
 	.halt			= pmac_halt,
 	.time_init		= pmac_time_init,
 	.get_boot_time		= pmac_get_boot_time,