omap: pandora: add fixed regulator for wlan

Instead of enabling the wifi module explicitly using GPIO, add a fixed
regulator and hook it to MMC host card power control. This way it will
only be enabled when SDIO subsystem wants to talk to it, saving power
(as done by Zoom boards).

Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>