riscv: Register System RAM as iomem resources

Add System RAM to /proc/iomem, various tools expect it such as kdump.
It is also needed for page_is_ram API which checks the specified address
whether registered as System RAM in iomem_resource list.

Signed-off-by: Zong Li <zong.li@sifive.com>
[Palmer: check MEMBLOCK_NOMAP]
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>