arch: riscv: smp: prevent nested #ifdef

The `CONFIG_PLIC_IRQ_AFFINITY` depends on `CONFIG_SMP` in the
Kconfig layer, nested #ifdef can be avoided for readability.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>