scripts: dts: dtlib: improve formatting of long arrays

Split long arrays into multiple lines to improve readability of the
output DTS file. A new line is started when the array data exceeds
80 characters.

Add a few test entries to verify the new behavior.

Note: the F821 linter suppression prevents flagging 'array_start' and
'array_newline' as undefined variables. This is because these variables
are initialized when an opening brace is output, which is necessarily
before any element in a byte or prop array. A sequence of markers not
following this pattern would indicate a bug in the DTS parsing code.

Signed-off-by: Luca Burelli <l.burelli@arduino.cc>