ARM: davinci_all_defconfig: enable RTC driver as module

This driver handles the RTC of most davinci SoCs.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Kevin Hilman <khilman@baylibre.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Karl Beldan <kbeldan@baylibre.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>