mmc: add sdhci-tegra driver for Tegra SoCs

SDHCI driver for Tegra. This driver plugs in as a new variant of
sdhci-pltfm, using the platform data structure passed in to specify the
GPIOs to use for card detect, write protect and card power enablement.

Original driver (of which only the header file is left):
Signed-off-by: Yvonne Yip <y@palm.com>

The rest, which has been rewritten by now:
Signed-off-by: Olof Johansson <olof@lixom.net>
Reviewed-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Chris Ball <cjb@laptop.org>