[fdtmem] Update to use the generic system memory map API

[fdtmem] Update to use the generic system memory map API

Provide an implementation of the system memory map API based on the
system device tree, excluding any memory outside the size of the
accessible physical address space and defining an in-use region to
cover the relocated copy of iPXE and the system device tree.

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