Add documentation for the SiFive implementation of the RISC-V Platform
Level Interrupt Controller (PLIC).  The PLIC connects global interrupt
sources to the local interrupt controller on each hart.
Signed-off-by: NPalmer Dabbelt <palmer@dabbelt.com>
[hch: various fixes and updates]
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>