omap: Fix spotty MMC voltages

As noted by Michał Mirosław <mirqus@gmail.com>, the voltages should
cover the supported voltage range, or support only one voltage.

As all these boards are using a GPIO to enable the power, chances
are that only 3.3V cards are supported on these boards.

Reported-by: Michał Mirosław <mirqus@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>