kirkwood: Add Marvell RTC to all machines with nothing else

kirkwood: Add Marvell RTC to all machines with nothing else

The Kirkwood SoCs all have an onchip RTC that can hold the time
over e.g. a reboot which will help if no NTP servers are available.

Create a kernel module package for the Marvell RTC, and add it to
all Kirkwood devices that do not have their own discrete
battery-backed RTC. Adding it to platforms with a proper RTC
is just surplus.

All Kirkwoods have at least one RTC so add RTC to the features
list for Kirkwood as well.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>