Add functions to access the maximum size we can read/write using
regmap_raw_read/write().

This helps drivers that need to know how much they can write with the
raw functions without problems. There are some devices (e.g. bmc150)
that have fifos as registers which need to be read in specific chunks
otherwise samples are dropped.
Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>