sandbox: Support setting up the other FDT for testing

sandbox: Support setting up the other FDT for testing

Provide a way to copy over the 'other' FDT when running tests. This loads
it and allocates memory for the copy, if not done already, then does the
copy.

Avoid using U-Boot's malloc() pool for these copies, at least for now,
since they are part of the test system.

Tidy up the cpu.c header files while here.

Signed-off-by: Simon Glass <sjg@chromium.org>