Add documentation on the RISC-V local interrupt controller, which is a
per-hart interrupt controller that manages all interrupts entering a
RISC-V hart.  This interrupt controller is present on all RISC-V systems.
Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>
[hch: minor cleanups]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>