spi: spi-fsl-dspi: Re-use one volatile regmap for both device types

spi: spi-fsl-dspi: Re-use one volatile regmap for both device types

max_register overrides anything in the volatile ranges, so we can get
away with sharing the same one for both types. In a later commit we'll
add more devices so this avoids adding even more duplication. Also
replace the max_register magic numbers with their register definitions
so it's clearer what's going on.

No functional changes.

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: James Clark <james.clark@linaro.org>
Link: https://patch.msgid.link/20250522-james-nxp-spi-v2-4-bea884630cfb@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>