regulator: giving regulator controlling gpios a non-empty label when used through the devicetree.

When the label is empty, it causes missing information and limits diagnostics
for instances such as 'cat /sys/kernel/debug/gpio'

Setting the label to the regulator supply_name will point to the device
using the gpio(s).

Signed-off-by: Nicholas Lowell <nlowell@lexmark.com>
Signed-off-by: Mark Brown <broonie@kernel.org>