kendryte/k210: Use sifive UART driver

The Kendryte K210 UARTHS is compatible with SiFive UART. So use the
sifive uart driver and remove the k210 uarths platform code.

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>