RISC-V: provide the correct value for kernstart

pmap_bootstrap() expects the kernel's physical load address, but we have
been providing the start of physical memory. This had the nice effect of
protecting the memory used by the SBI runtime firmware, but now that we
have alternate means of achieving that, we should provide the correct
value. This will free up any memory between the SBI firmware and the
kernel for allocation.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D24156