Add optional /bits/ parsing

[tinnedkarma]

Summary

Add support for parsing the /bits/ directive in devicetree cell arrays, which allows specifying element sizes other than the default 32-bit.

Description

Hi, I'm using pydevicetree as project dependency for memory constrained devices.

Although not in the v0.4 specs, the device tree compiler supports /bits/ directive specified here which I use to reduce the device tree blob size.

I've noticed that the pydevicetree parser fails if the /bits/ directive is present in the dts files.

This PR adds new optional tokens/elements to cell_array parser and updates the CellArray class to support the new bits parameter. By default, the new CellArray member bits is 32 bits.