am57x: Move CONS_INDEX to Kconfig

- Move the CONS_INDEX selection out of CONFIG_SYS_EXTRA_OPTIONS and
      into Kconfig proper.
    - Edit the relevant am57x configs to remove the now unneeded
      CONFIG_SYS_EXTRA_OPTIONS.

Signed-off-by: Daniel Allred <d-allred@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>