ARM: SMDK6410: Specify no GPIO for B_PWR_5V regulator

Since the fixed voltage regulator grew support for GPIO based
enables and GPIO 0 is valid on some systems we need to specify
that there is no valid GPIO enable control.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>