soc: riscv: sifive-freedom: reorganize SoC folder

Reorganized as follows:

- Created a new SiFive Freedom family
- Created 3 new series: E300/E500/E700
- Created Socs within each series (e.g. E340)

Also moved out of riscv-privileged folder.

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