spi: master driver to enable RTC on ICPDAS LP-8841

ICP DAS LP-8841 contains a DS-1302 RTC. This driver provides an SPI
master which makes the RTC usable. The driver is not supposed to work
with anything else.

The driver uses the standard MicroWire half-duplex transfer timing.
Master output is set on low clock and sensed by the RTC on the rising
edge. Master input is set by the RTC on the trailing edge and is sensed
by the master on low clock.

Signed-off-by: Sergei Ianovich <ynvich@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>