hw/intc: ibex_plic: Honour source priorities

This patch follows what commit aa4d30f6 "riscv: plic: Honour source
priorities" does and ensures that the highest priority interrupt will be
serviced first.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Cc: Jessica Clarke <jrtc27@jrtc27.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <a697ca8a31eff8eb18a88e09a28206063cf85d48.1595655188.git.alistair.francis@wdc.com>