fsl_dspi: Introduce DT bindings for CS-SCK and SCK-CS delays

Communication with some SPI slaves just won't cut it if these delays
(before the beginning, and after the end of a transfer) are not added to
the Chip Select signal.

These are a straight copy from Linux:
Documentation/devicetree/bindings/spi/spi-fsl-dspi.txt
drivers/spi/spi-fsl-dspi.c

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
[Rebased]
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>

doc/device-tree-bindings/spi/spi-mcf-dspi.txt

@@ -13,6 +13,10 @@ Optional properties:
 - ctar-params: CTAR0 to 7 register configuration, as an array
   of 8 integer fields for each register, where each register
   is defined as: <fmsz, pcssck, pasc, pdt, cssck, asc, dt, br>.
+- fsl,spi-cs-sck-delay: a delay in nanoseconds between activating chip
+  select and the start of clock signal, at the start of a transfer.
+- fsl,spi-sck-cs-delay: a delay in nanoseconds between stopping the clock
+  signal and deactivating chip select, at the end of a transfer.
 
 Example: