riscv: rv32_defconfig: Update the defconfig

Update the rv32_defconfig:
 - Add 'CONFIG_DEVTMPFS_MOUNT=y' to match the RISC-V defconfig
 - Add CONFIG_HW_RANDOM=y and CONFIG_HW_RANDOM_VIRTIO=y to enable
   VirtIORNG when running on QEMU

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>