riscv: Enable GENERIC_IRQ_SHOW_LEVEL

The interrupt controllers on riscv support both edge and level triggered
interrupts, it's useful to provide that information in /proc/interrupts.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>