mmc: host: sdhci: tegra: Support runtime enable/disable of clock

mmc: host: sdhci: tegra: Support runtime enable/disable of clock

Currently the SD card clock is running any time the AP is not in
LP0. This patch allows the clock to only be enabled when the card
is being accessed.

Based on an nVidia patch:
http://nv-tegra.nvidia.com/gitweb/?p=linux-2.6.git;a=commit;h=8c0f23b7e65ebdb6d1c160517e9ba509b16ec83f

Change-Id: I1b611622ef0545a3309214e94bbc76481aa3a721
Signed-off-by: Greg Meiste <w30289@motorola.com>
Conflicts:

	drivers/mmc/host/sdhci-tegra.c

Signed-off-by: Greg Meiste <w30289@motorola.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>