hvc: Convert to using interrupts instead of opal events

Convert the opal hvc driver to use the new irqchip to register for
opal events. As older firmware versions may not have device tree
bindings for the interrupt parent we just use a hardcoded hwirq based
on the event number.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>