openrisc: Use the new GENERIC_IRQ_MULTI_HANDLER

It appears that openrisc copied arm64's GENERIC_IRQ_MULTI_HANDLER code
(which came from arm).  I wanted to make this generic so I could use it
in the RISC-V port.  This patch converts the openrisc code to use the
generic version.

Acked-by: Stafford Horne <shorne@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>