regulator: gpio-regulator: doc: Fix wrong key gpio-enable

Wrong description in binding document.
Doc use "gpio-enable", but code use "enable-gpio"

In drivers/regulator/gpio-regulator.c
	config->enable_gpio = of_get_named_gpio(np, "enable-gpio", 0);

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>