riscv: mm: fix wrong phys_ram_base value for RV64

Currently, if 64BIT and !XIP_KERNEL, the phys_ram_base is always 0,
no matter the real start of dram reported by memblock is.

Fixes: 6d7f91d9 ("riscv: Get rid of CONFIG_PHYS_RAM_BASE in kernel physical address conversion")
Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Alexandre Ghiti <alex@ghiti.fr>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>