mfd: Convert 88pm860x to use regmap api

Convert the 88pm860x normal bank register read/write to
use the register map API.

Signed-off-by: Jett.Zhou <jtzhou@marvell.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
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