RISC-V: Move to the new GENERIC_IRQ_MULTI_HANDLER handler

The old mechanism for handling IRQs on RISC-V was pretty ugly: the arch
code looked at the Kconfig entry for our first-level irqchip driver and
called into it directly.

This patch uses the new generic IRQ handling infastructure, which
essentially just deletes a bunch of code.  This does add an additional
load to the interrupt latency, but there's a lot of tuning left to be
done there on RISC-V so I think it's OK for now.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>