rtc: davinci: add driver model support

Update the driver to support the device tree and the driver model.
The read / write helpers in rtc_ops allow access to scratch registers
only. The offset parameter is added to the address of the scratch0
register.

Support for non-DM has been removed as there were no users.

Signed-off-by: Dario Binacchi <dariobin@libero.it>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210602203805.11494-8-dariobin@libero.it