ARM: ux500: regulators: Add mask for configuration

There is already before a register mask in the regulator driver
to allow some bits of a register to be initialized. The register
value is defined in the board configuration. This patch puts a
mask in the board configuration to specify which bits should
actually be altered. The purpose with this patch is to avoid
future mistakes when updating the allowed bits in the regulator
driver.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>