powerpc: mpc85xx: Use binman to embed dtb inside U-Boot

Below is the sequence to embed dtb inside U-Boot,
1. Remove bootpg and resetvec section if needed
2. Append dtb
3. Append bootpg and resetvec section back if removed in step 1

Above procedure is required only when CONFIG_MPC85xx and
CONFIG_OF_SEPARATE are defined.

Add new config CONFIG_MPC85XX_HAVE_RESET_VECTOR to indicate that
image has resetvec section. Step 1 and step 3 described above are
required only if this config is y.

Signed-off-by: Jagdish Gediya <jagdish.gediya@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: York Sun <york.sun@nxp.com>