hw/intc/ibex_plic: Clear the claim register when read

hw/intc/ibex_plic: Clear the claim register when read

After claiming the interrupt by reading the claim register we want to
clear the register to make sure the interrupt doesn't appear at the next
read.

This matches the documentation for the claim register as when an interrupt
is claimed by a target the relevant bit of IP is cleared (which we already
do): https://docs.opentitan.org/hw/ip/rv_plic/doc/index.html

This also matches the current hardware.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 68d4575deef2559b7a747f3bda193fcf43af4558.1604629928.git.alistair.francis@wdc.com