mxs: power: Change sequence of enabling DCDC switches

The imx28 uses following voltage supplies hierarchy:

VDD_5V (VDD_BAT) -> VDDIO -> VDDA -> VDDMEM
			     \-----> VDDD

One shall first enable DCDC on the parent source (VDDIO) and then
follow with its children.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>