rtc: omap: Support scratch registers

Register an nvmem device to expose the 3 scratch registers (total of 12
bytes) to both userspace and kernel space.

Reviewed-by: Sekhar Nori <nsekhar@ti.com>
Tested-by: Keerthy <j-keerthy@ti.com>
Reviewed-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>