regulator: 88pm800: fix warning same module names

When building with CONFIG_MFD_88PM800 and CONFIG_REGULATOR_88PM800
enabled as loadable modules, we see the following warning:

warning: same module names found:
  drivers/regulator/88pm800.ko
  drivers/mfd/88pm800.ko

Rework so that the file is named 88pm800-regulator.

Signed-off-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>