RISC-V: Rename riscv_of_processor_hart to riscv_of_processor_hartid

It's a bit confusing exactly what this function does: it actually
returns the hartid of an OF processor node, failing with -1 on invalid
nodes.  I've changed the name to _hartid() in order to make that a bit
more clear, as well as adding a comment.

Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
[Atish: code comment formatting update]
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>