Fix IGEPv2 second MMC channel power supply

Commit 72f381ba (omap3: Remove VMMC2 regulator on IGEP v2)
removed an unused regulator entry, but left the second MMC channel
(used by the Libertas WLAN module) without link to power regulator.
This causes the SDIO module to fail being detected.

This patch adds the two regulators that actually feed the WLAN module
(1v8 from the TWL4030 VIO LDO, and a fixed 3v3). With that patch, the
second channel is properly detected. Details of the power supply
implementation were kindly provided by Enric Balletbo i Serra.

Also change vmmc1 to use symbolic names instead of direct device
reference.

Tested on an IGEPv2 Rev-B.

Signed-off-by: Marc Zyngier <maz@misterjones.org>
Acked-by: Enric Balletbo i Serra <eballetbo@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>