arm64: Set CONFIG_MULTI_IRQ_HANDLER

arm has an optional MULTI_IRQ_HANDLER, which arm64 copied but didn't make
optional.  The multi irq handler infrastructure has been copied to generic
code selectable with a new config symbol. That symbol can be selected by
randconfig builds and can cause build breakage.

Introduce CONFIG_MULTI_IRQ_HANDLER as an intermediate step which prevents
the core config symbol from being selected. The arm64 local config symbol
will be removed once arm64 gets converted to the generic code.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Link: https://lkml.kernel.org/r/20180404043130.31277-2-palmer@sifive.com