ARM: ux500: Setup external regulator supply #2 for HREF v20 boards

v20 HREFs require non-standard configuration of EXT_SUPPLY2 to
function correctly (specific information is commented). Here we make
use of the recently added mechanism to adapt initialisation values
for such use-cases.

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