rtc: at91sam9: rework the Kconfig description

Remove all references to AT91CAP9 SoC which has been removed.
Rework help message to remove any specific references to AT91SAM9 SoCs.
State that RTC_DRV_AT91SAM9_RTT and RTC_DRV_AT91SAM9_GPBR options are only
used when booting non DT boards.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Acked-by: Johan Hovold <johan@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>