armv8: layerscape: clean exported symbols in spintable.S

armv8: layerscape: clean exported symbols in spintable.S

Add a new variable secondary_boot_code_start, which holds a pointer to
the start of the spin table code. This will help to relocate the code
section. While at it, move the size variable from the end to the
beginning so there is a common section for the variables. Remove any
other symbols.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>