regmap: mmio: Add missing MODULE_DESCRIPTION()

There were already several commits to add module descriptions to regmap
modules.  But this one was still missing:

WARNING: modpost: missing MODULE_DESCRIPTION() in drivers/base/regmap/regmap-mmio.o

Signed-off-by: Luis Henriques <luis@igalia.com>
Link: https://patch.msgid.link/20250728150829.11890-1-luis@igalia.com


Signed-off-by: Mark Brown <broonie@kernel.org>