RISC-V: defconfig: enable MMC & SPI for RISC-V

Currently, riscv upstream defconfig doesn't let you boot
through userspace if rootfs is on the SD card.

Let's enable MMC & SPI drivers as well so that one can boot
to the user space using default config in upstream kernel.

While here, enable automatic mounting of devtmpfs to simplify
kernel testing with minimal root filesystems. (pjw)

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
[paul.walmsley@sifive.com: mention the DEVTMPFS_MOUNT change in the
 patch description]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>