kernel: deprecate support for the generic "linux,part-probe" DT binding

It has been rejected upstream and instead a nice/more generic solution
has been implemented. It's possible now to describe partitions format
using "compatible" DT string.

No OpenWrt target uses "linux,part-probe" anymore, leave it only in case
some forks need it. It will be dropped with support for new kernels.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>