drivers/rtc/rtc-tegra.c: convert to DT driver

After Kernel 3.7, the Tegra device will only support booting with DT.
The legacy Tegra RTC platform driver will not work anymore.  This patch
converts the driver to support DT.

Signed-off-by: Joseph Lo <josephl@nvidia.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Joseph Lo <josephl@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>