The Ibex core contains a PLIC that although similar to the RISC-V spec
is not RISC-V spec compliant.

This patch implements a Ibex PLIC in a somewhat generic way.

As the current RISC-V PLIC needs tidying up, my hope is that as the Ibex
PLIC move towards spec compliance this PLIC implementation can be
updated until it can replace the current PLIC.
Signed-off-by: NAlistair Francis <alistair.francis@wdc.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>