irq: Port the ARM IRQ drivers to GENERIC_IRQ_MULTI_HANDLER

GENERIC_IRQ_MULTI_HANDLER is incompatible with MULTI_IRQ_HANDLER because
they define the same symbols.  Multiple generic irqchip drivers select
MULTI_IRQ_HANDLER, which is now defined on all architectures that
provide set_handle_irq().  This patch selects GENERIC_IRQ_MULTI_HANDLER
for all drivers that used to select MULTI_IRQ_HANDLER, but only when
MULTI_IRQ_HANDLER doesn't exist.  I'll then convert every architecture
over from MULTI_IRQ_HANDLER to GENERIC_IRQ_MULTI_HANDLER before removing
the extra MULTI_IRQ_HANDLER scaffolding.

CC: Shea Levy <shea@shealevy.com>
CC: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>