riscv: Fix irq_work when SMP is disabled

riscv: Fix irq_work when SMP is disabled

irq_work is triggered via an IPI, but the IPI infrastructure is not
included in uniprocessor kernels. As a result, irq_work never runs.
Fall back to the tick-based irq_work implementation on uniprocessor
configurations.

Fixes: f00d8c70453f ("riscv: Support irq_work via self IPIs")
Signed-off-by: Samuel Holland <samuel@sholland.org>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220430030025.58405-1-samuel@sholland.org
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>