ARM: dts: imx6*-hummingboard2: rework regulators

Name the regulators according to the voltage rails they are feeding in
the schematic. The USB 2-4 regulators are fixed regulators and are always
on, as they are not a VBUS supply for a single USB host port on the i.MX6,
but supply VBUS to ports behind a USB hub and there is currently no way
to model this in mainline.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>