arm/dt: Fix ventana SDHCI power-gpios

Ventana uses the same SDHCI GPIOs as Seaboard; PI6 (70) is the power GPIO
for the SD port, and there is no power GPIO for the MMC chip.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>