regmap: Annotate that MMIO implies fast IO

Document that using the MMIO helpers will automatically enable the
'fast_io' parameter. This makes the used locking scheme more transparent
and avoids superfluous setting of this parameter in drivers.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://patch.msgid.link/20250725110337.4303-2-wsa+renesas@sang-engineering.com


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