RISCV: PLATFORM_DEFAULT implementation

[nick]

Summary

Set up a default plaform for RISCV. This is useful in the sense that it
moves toward the direction of being able to assume an instance of a
PLATFORM exists. If PLATFORM and PLATFORM_DEFAULT become standard kernel
options then we could remove our "#ifdef PLATFORM"'s as we can then fall
back on the default platform functions.

Sponsored by: Axiado

[philip]

Looks good to me.