regulator: lm363x: Use generic DT property name for external control pins

Vpos and Vneg LDOs can be enabled or disabled by external GPIOs.
Use general DT property 'enable-gpios' for this usage.
Two enable pins are differentiable by selecting the index number.

Signed-off-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>