irq: Add CONFIG_GENERIC_IRQ_MULTI_HANDLER

It looks like the arm irqchip registration mechanism has been copied
into a handful of ports, including arm64 and openrisc.  I want to use
this in the RISC-V port, so I thought it would be good to make this
generic instead.

This patch copies the arm definition of CONFIG_MULTI_IRQ_HANDLER into
kernel/irq under CONFIG_GENERIC_MULTI_IRQ_HANDLER.  This patch is
currently all dead code, but it will be enabled in the various other
architectures in subsequent patches.

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