RISC-V: Add address map dumper

Add support for dumping the kernel address space layout to the console.
User can enable CONFIG_DEBUG_VM to dump the virtual memory region into
dmesg buffer during boot-up.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
[paul.walmsley@sifive.com: dropped .init/.text/.data/.bss prints;
 added PCI legacy I/O region display]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>