doc: Improve layout of Zephyr-specific DTS properties table

The layout of this table unreasonably assigns narrow widths for
for column(s) with lots of content, and wise-versa.

Define relative column widths to be more in sync with column
content. This will potentially produce more compact table, making
it more readable, clearer and more visually pleasing.

Signed-off-by: Aleksandar Markovic <aleksandar.markovic.sa@gmail.com>