drivers/rtc/rtc-tegra.c: use managed rtc_device_register()

drivers/rtc/rtc-tegra.c: use managed rtc_device_register()

Use devm_rtc_device_register() for registering RTC device.  This will
reduce the code for unregistering RTC device in cleanup path and remove
the implementation of remove callback of platform driver.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Thierry Reding <thierry.reding@avionic-design.de>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Cc: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>