riscv: Force flat memory model with no-mmu

Compilation errors trigger if ARCH_SPARSEMEM_ENABLE is enabled for
a nommu kernel. Since the sparsemem model does not make sense anyway
for the nommu case, do not allow selecting this option to always use
the flatmem model.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>