drivers: intc_plic: claim interrupt from hart that serviced it

The plic has a very simple mechanism to claim an interrupt as well as to
complete and clear it. The same register is read from/ written to to
achieve this.

Get the ID of the HART that serviced the interrupt and write to the
claim complete register in the correct context
Signed-off-by: Conor Paxton <conor.paxton@microchip.com>