rtc: ds1302: rewrite using SPI

DS1302 is an half-duplex SPI device. The driver respects this fact now.
Pin configurations should be implemented using SPI subsystem.

Signed-off-by: Sergei Ianovich <ynvich@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>