drivers/of: Improve documentation for match_string

The description of the function now explicitly states that it's
an *exact* match for the given string (i.e. not a submatch). It also
better states all the possible return values.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>
Link: https://lore.kernel.org/r/20240911204938.9172-1-mikisabate@gmail.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>