riscv: Fix Canaan Kendryte K210 device tree

Remove the clocks property from the cpu and clint nodes as these are
ignored. Also remove the clock-frequency property from the cpu nodes as
riscv relies on the timebase-frequency property.

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>