tty/hvc: Use opal irqchip interface if available

Update the hvc driver to use the OPAL irqchip if made available by the
running firmware. If it is not present, the driver falls back to the
existing OPAL event number.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>