openrisc: Set CONFIG_MULTI_IRQ_HANDLER

arm has an optional MULTI_IRQ_HANDLER, which openrisc 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 openrisc local config
symbol will be removed once openrisc 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/20180404043130.31277-3-palmer@sifive.com