spi: cadence: Remove Kconfig dependency

Remove Kconfig dependency and enable driver for
all ARCHs.
Also update help description.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Mark Brown <broonie@kernel.org>