interrupt: Convert RISC-V plic to use multi-level irq support

Utilize the multi-level irq infrastructure and replace custom handling
for PLIC on riscv-privilege SoCs.  The old code offset IRQs in drivers
and various places with RISCV_MAX_GENERIC_IRQ.  Instead utilize Zephyr's
encoded IRQ and replace offsets in drivers with the IRQ define from DTS.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>