board/orangepi-zero-plus2: switch to GPT for PARTLABEL support

board/orangepi-zero-plus2: switch to GPT for PARTLABEL support

Patch that pins mmc indexes was not accepted to mainline kernel. Drop that
patch and switch to GPT to use partition labels. For GPT the name of the
partition in genimage.cfg is used as the label for that partition. Note
that the default GPT partition table location conflicts with the SPL
location, so move GPT table after bootloaders.

Signed-off-by: Sergey Matyukevich <geomatsi@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>