clk: k210: Move k210 clock out of its own subdirectory

Now that we have only one clock driver, we don't need to have our own
subdirectory. Move the driver back with the rest of the clock drivers.

The MAINTAINERS for kendryte pinctrl is also fixed since it has always been
wrong.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>