SPI: Renamed spi-zynq as spi-cadence

Since spi is a cadence IP, renamed spi-zynq as spi-cadence.
The differences between the cadence IP and zynq IP are
only a few configurable options in cadence.
Hence, using the name cadence seems appropriate.
This patch renames spi-zynq.c to spi-cadence.c and changes
all the zynq prefixes to cdns. Zynq to Cadence change is
reflected in Kconfig, Makefile, defconfig and dts of
zc770_xm010 and zc770_xm013.

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>