k210: use the board vendor name rather than the marketing name

"kendryte" is the marketing name for the K210 RISC-V SoC produced by
Canaan Inc. Rather than "kendryte,k210", use the usual "canaan,k210"
vendor,SoC compatibility string format in the device tree files and
use the SoC name for file names.
With these changes, the device tree files are more in sync with the
Linux kernel DTS and drivers, making uboot device tree usable by the
kernel.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>