[POWERPC] fix cell pmu initialisation

[Benjamin Herrenschmidt]

> 1. export a method from cbe_regs.c to find out whether the pmd 
>    regs are available and test that in cbe_init_pm_irq().
> 2. take you patch as is, with a comment hinting to fix it up
>    when needed.
> 3. change cbe_init_pm_irq to be called explicitly from cbe_regs_init(),
>    which already scans the device tree correctly.

cbe_regs_init() is called from setup_arch(), which is _very_ early.

cbe_init_pm_irq() must be called much later as it requests irqs.

Right now, we should stick to the machine_is() solution. We'll look into
doing something better eventually when the need occurs.

Ben.