ARM: ux500: Supply external regulator names for Snowball's DT

Regulator names are platform independent, so they need to be applied to
the base level platform DTS files *.dts. Here we're supplying the names
for the newly described AB8500 external regulators.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>