Re-factor the console-related code into fdt_console.[ch]

Refactor the "platform" defitions into fdt_platform.h

risc-v: MULTIPROCESSOR support

Add MULTIPROCESSOR support for RISC-V, but leave disabled for the moment
as it's not 100% stable.

Some other improvements to spl and cpu identification / reporting.

RISC-V support that works on QEMU with a single hart.

Thanks for Simon Burge for plic(4).