riscv: Use vendor name for K210 SoC support

Rename configuration options and directories related to the Kendryte
K210 SoC to use the SoC vendor name (canaan) instead of the "kendryte"
branding name.

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