overlays: spi-rtc: Add ds3232 and ds3234

Extend the spi-rtc overlay to support the ds3232 and ds3234 RTCs, as
well as adding parameters to select difference SPI controllers and
chip selects.

N.B. The default CS is now active-low - use the "cs_high" parameter to
override this.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>