ipq806x: 5.15: add boot-partitions binding to fix block warning

ipq806x: 5.15: add boot-partitions binding to fix block warning

The patch has changed implementation and now the binding has changed.
Replace the old binding with boot-partitions and reimplement the
definition with the new definition.
The new definition is:
<offset1 size1 offset2 size2 offset3 ...>
and now supports sparsed patch.

Also add missing binding in some dts and add the backup boot partition
to the boot-partitions list.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>