powerpc/eSDCH: Specify voltage for T4240QDS

Freescale T4240QDS reference board has extra voltage shifters added
to allow 3.3V operation, so add 3.3v voltage support for T4240QDS.
1.8v and 3.3v is recommand for eMMC and SDHC card.

Signed-off-by: Haijun Zhang <haijun.zhang@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>