ARMV7: OMAP: Overo: Autodetect presence/absence of transceiver on mmc2

An upcoming version of Overo uses a Wifi/BT module with 1.8V signaling,
eliminating the need for an external transceiver to handle the level
shifting.  This patch detects whether an external transceiver is present
and adjusts the pinmux settings as appropriate.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>