Add initial support for the SiFive FU540-C000 SoC. FU740-C000 is built
around the SiFIve U7 Core Complex and a TileLink interconnect.

This file is expected to grow as more device drivers are added to the
kernel.
Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>