riscv: Update Canaan Kendryte K210 device tree

Update the Canaan Kendryte K210 base device tree k210.dtsi to define
all supported peripherals of the SoC, their clocks and reset lines.
The device tree file k210.dts is renamed to k210_generic.dts and
becomes the default value selection of the configuration option
SOC_CANAAN_K210_DTB_BUILTIN_SOURCE. No device beside the serial console
is defined by this device tree. This makes this generic device tree
suitable for use with a builtin initramfs with all known K210 based
boards.

These changes result in the K210_CLK_ACLK clock ID to be unused and
removed from the dt-bindings k210-clk.h header file.

Most updates to the k210.dtsi file come from Sean Anderson's work on
U-Boot support for the K210.

Cc: Rob Herring <robh@kernel.org>
Cc: devicetree@vger.kernel.org
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>