soc: riscv: telink_b91: reorganize SoC folder

Reorganize following the hierarchy found in the vendor website [1]:

- SoC Family: Telink TLSR
- SoC series: TLSR951X
- SoC: TLSR9518

Also split out from riscv-privileged folder. Note that B91 was the name
of a starter kit [2].

[1]: http://wiki.telink-semi.cn/wiki/chip-series/TLSR951x-Series/
[2]: https://wiki.telink-semi.cn/wiki/Hardware/


     B91_Generic_Starter_Kit_Hardware_Guide/

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>