riscv: Add Canaan Kendryte K210 SD card defconfig

The nommu_k210_defconfig default configuration allows booting a Canaan
Kendryte K210 SoC based boards using an embedded intramfs cpio file.
Modifying this configuration to enable support for the board SD card is
not trivial for all users. To help beginners getting started with these
boards, add the nommu_k210_sdcard_defconfig default configuration file
to set all configuration options necessary to use the board mmc-spi sd
card for the root file system.

This new configuration adds support for the block layer, the mmc-spi
driver and modifies the boot options to specify the rootfs device as
mmcblk0p1 (first partition of the sd card block device). The ext2 file
system is selected by default to encourage its use as that results in
only about 4KB added to the kernel image size. As ext2 does not have
journaling, the boot options specify a read-only mount of the file
system. Similarly to the smaller nommu_k210_defconfig, this new default
configuration disables virtual terminal support to reduce the kernel
image size.

The default device tree selected is unchanged, specifying the simple
"k210_generic" device tree file. The user must change this setting to
specify the device tree suitable for the board being used
(sipeed_maix_bit, sipeed_maix_dock, sipeed_maix_go, sipeed_maixduino or
canaan_kd233).

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