riscv: Fix print_vm_layout build error if NOMMU

arch/riscv/mm/init.c: In function ‘print_vm_layout’:
arch/riscv/mm/init.c:68:37: error: ‘FIXADDR_START’ undeclared (first use in this function);
arch/riscv/mm/init.c:69:20: error: ‘FIXADDR_TOP’ undeclared
arch/riscv/mm/init.c:70:37: error: ‘PCI_IO_START’ undeclared
arch/riscv/mm/init.c:71:20: error: ‘PCI_IO_END’ undeclared
arch/riscv/mm/init.c:72:38: error: ‘VMEMMAP_START’ undeclared
arch/riscv/mm/init.c:73:20: error: ‘VMEMMAP_END’ undeclared (first use in this function);

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