mfd: Avoid to use constraint name in 88pm860x regulator driver

Avoid to use constraint name in regulator driver. So use regulator id is used
instead in platform driver.

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Cc: Liam Girdwood <lrg@slimlogic.co.uk>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>