riscv: Rename the FE310 SoC to Sifive Freedom

FE310 is the name of one SoC out of a range of products in the SiFive
Freedom line. The FE310 SoC port in Zephyr is compatible with all of
these products, so rename the SoC to SiFive Freedom

Signed-off-by: Nathaniel Graff <nathaniel.graff@sifive.com>