irqchip: RISC-V Local Interrupt Controller Driver

This patch adds a driver that manages the local interrupts on each
RISC-V hart, as specifiec by the RISC-V supervisor level ISA manual.
The local interrupt controller manages software interrupts, timer
interrupts, and hardware interrupts (which are routed via the
platform level interrupt controller).  Per-hart local interrupt
controllers are found on all RISC-V systems.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>