ARM: dts: imx6qdl-hummingboard: add SD card regulator

Add the SD card regulator, which allows the SD card power to be switched
on/off.

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>