kendryte/k210: remove sysctl code

Directly implement frequency discovery, making the sysctl code
unnecessary. While at it, Move all macro definitions from platform.c
into platform.h and cleanup that file, removing the need for the
Apache 2.0 license and Canaan Inc copyright.

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>