mmc: tegra: Support module reset

The device tree binding for the SDHCI controller found on Tegra SoCs
specifies that a reset control can be provided by the device tree. No
code was ever added to support the module reset, which can cause the
driver to try and access registers from a module that's in reset. On
most Tegra SoC generations doing so would cause a hang.

Note that it's unlikely to see this happen because on most platforms
these resets will have been deasserted by the bootloader. However the
portability can be improved by making sure the driver deasserts the
reset before accessing any registers.

Since resets are synchronous on Tegra SoCs, the platform driver needs
to implement a custom ->remove() callback now to make sure the clock
is disabled after the reset is asserted.

Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>