[riscv] Support mapping I/O devices outside of the identity map

[riscv] Support mapping I/O devices outside of the identity map

With the 64-bit paging schemes (Sv39, Sv48, and Sv57), we identity-map
as much of the physical address space as is possible.  Experimentation
shows that this is not sufficient to provide access to all I/O
devices.  For example: the Sipeed Lichee Pi 4A includes a CPU that
supports only Sv39, but places I/O devices at the top of a 40-bit
address space.

Add support for creating I/O page table entries on demand to map I/O
devices, based on the existing design used for x86_64 BIOS.

Signed-off-by: Michael Brown <mcb30@ipxe.org>