modules: hal_nordic: nrf-regtool: enforce in-tree devicetree package

It looks like nrf-regtool depends on the
https://pypi.org/project/devicetree/ Python package, which contains an
old copy (https://github.com/zephyrproject-rtos/python-devicetree

) of
the in-tree devicetree package.

Because no major changes have happened in the devicetree scripts, this
likely went unnoticed. However, realized today that such old package is
not able to parse bindings with multiple buses (e.g. in I3C, where we
have `bus: [i3c, i2c]`).

While a proper solution is discussed, inject `PYTHONPATH` with the
in-tree library.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>