mfd: hi6421-spmi-pmic: Cleanup drvdata to only include regmap

There are lots of fields in struct hi6421_spmi_pmic that aren't
used. As a matter of fact, only regmap is needed.

So, drop the struct as a whole, and set regmap as the drvdata.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Acked-by: Mark Brown <broonie@kernel.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/1828cb783b1ebca0b98bf0b3077d8701adb228f7.1630586862.git.mchehab+huawei@kernel.org