AT91: rtc: enable built-in RTC in Kconfig for at91sam9g45 family

Enable built-in RTC IP in Kconfig and modify comments and help messages.
RTT as RTC is still available but should not be selected in common case.

Reported-by: Yegor Yefremov <yegor_sub1@visionsystems.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>