mfd: cros_ec: spi: Add a DT property to delay asserting the CS

Some ECs need a little time for waking up before they can accept
SPI data at a high speed. Add a "google,cros-ec-spi-pre-delay"
property to the DT binding to configure this.

If this property isn't set, then no delay will be added. However,
if set it will cause a delay equal to the value passed to it to
be inserted at the beginning of a transaction.

Signed-off-by: Alexandru M Stan <[e-mail]>
Reviewed-by: Doug Anderson <[e-mail]>
Signed-off-by: Chris Zhong <[e-mail]>
Signed-off-by: Javier Martinez Canillas <[e-mail]>
Tested-by: Heiko Stuebner <[e-mail]>
Acked-by: Lee Jones <[e-mail]>
Acked-by: Olof Johansson <[e-mail]>
Signed-off-by: Lee Jones <[e-mail]>

Documentation/devicetree/bindings/mfd/cros-ec.txt

···
 - reg: SPI chip select
 
 Optional properties (SPI):
+- google,cros-ec-spi-pre-delay: Some implementations of the EC need a little
+  time to wake up from sleep before they can receive SPI transfers at a high
+  clock rate. This property specifies the delay, in usecs, between the
+  assertion of the CS to the start of the first clock pulse.
 - google,cros-ec-spi-msg-delay: Some implementations of the EC require some
   additional processing time in order to accept new transactions. If the delay
   between transactions is not long enough the EC may not be able to respond