i.MX31: Make the SPI bus and chip select configurable for MC13783

The i.MX31 has three SPI buses and each bus has several chip selects
and the MC13783 chip can be connected to any of these. The current
RTC driver for MC13783 is hardcoded for CSPI2/SS2.

This patch makes make MC13783 SPI bus and chip select configurable
via CONFIG_MC13783_SPI_BUS and CONFIG_MC13783_SPI_CS.

Signed-off-by: Magnus Lilja <lilja.magnus@gmail.com>