.mailmap: map old addresses to their up-to-date counterparts

This essentially maps @free-electrons.com to @bootlin.com addresses.

Signed-off-by: João Marcos Costa <joaomarcos.costa@bootlin.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>