riscv: Select required drivers for Kendryte SOC

This patch selects drivers required for the Kendryte K210 SOC.
Since K210 SoC based boards do not provide a device tree, this patch
also enables the BUILTIN_DTB option.

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