riscv: Add Kendryte K210 SoC support

riscv: Add Kendryte K210 SoC support

Add support for the Kendryte K210 RISC-V SoC. For now, this support
only provides a simple sysctl driver allowing to setup the CPU and
uart clock. This support is enabled through the new Kconfig option
SOC_KENDRYTE and defines the config option CONFIG_K210_SYSCTL
to enable the K210 SoC sysctl driver compilation.

The sysctl driver also registers an early SoC initialization function
allowing enabling the general purpose use of the 2MB of SRAM normally
reserved for the SoC AI engine. This initialization function is
automatically called before the dt early initialization using the flat
dt root node compatible property matching the value "kendryte,k210".

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
[Palmer: Add missing endmenu in Kconfig.socs]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>