RTC: Add JZ4740 RTC driver

Add support for the RTC unit on JZ4740 SoCs.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Paul Gortmaker <p_gortmaker@yahoo.com>
Cc: rtc-linux@googlegroups.com
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Cc: Alessandro Zummo <a.zummo@towertech.it>,
Patchwork: https://patchwork.linux-mips.org/patch/1424/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>