mmc: core: Move regulator helpers to separate file

The mmc regulator helper functions, are placed in the extensive core.c
file.  In a step towards trying to create a better structure of files,
avoiding too many lines of code per file, let's move these helpers to a new
file, regulator.c.

Moreover, this within this context it makes sense to also drop the export
of mmc_vddrange_to_ocrmask(), but instead let's make it internal to the mmc
core.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>