RISC-V: Make NUMA depend on SMP

In theory these are orthogonal, but in practice all NUMA systems are
SMP. NUMA && !SMP doesn't build, everyone else is coupling them, and I
don't really see any value in supporting that configuration.

Fixes: 4f0e8eef772e ("riscv: Add numa support for riscv64 platform")
Suggested-by: Andrew Morton <[e-mail]>
Suggested-by: Atish Patra <[e-mail]>
Reported-by: Kefeng Wang <[e-mail]>
Tested-by: Kefeng Wang <[e-mail]>
Signed-off-by: Palmer Dabbelt <[e-mail]>

arch/riscv/Kconfig

···
 # Common NUMA Features
 config NUMA
 	bool "NUMA Memory Allocation and Scheduler Support"
+	depends on SMP
 	select GENERIC_ARCH_NUMA
 	select OF_NUMA
 	select ARCH_SUPPORTS_NUMA_BALANCING