riscv: Allow device trees to be built into the kernel

Some systems don't provide a useful device tree to the kernel on boot.
Chasing around bootloaders for these systems is a headache, so instead
le't's just keep a device tree table in the kernel, keyed by the SOC's
unique identifier, that contains the relevant DTB.

This is only implemented for M mode right now. While we could implement
this via the SBI calls that allow access to these identifiers, we don't
have any systems that need this right now.

Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>