dt: add helper for phandle allocation

dt: add helper for phandle allocation

Phandle references work by having 2 pieces:

  - a "phandle" 1-cell property in the device tree node
  - a reference to the same value in a property we want to point
    to the other node

To generate the 1-cell property, we need an allocation mechanism that
gives us a unique number space. This patch adds an allocator for these
properties.

Signed-off-by: Alexander Graf <agraf@suse.de>