mfd: cros_ec: spi: Add delay for asserting CS

Some ECs need a little time for waking up before they can accept
SPI data at a high speed. This is configurable via a DT property
"google,cros-ec-spi-pre-delay".

This patch makes the cros_ec_spi driver to cause a delay before
the beginning of a SPI transaction, to make sure that the EC has
already woken up, if the property has been defined in the DTS.

Signed-off-by: Alexandru M Stan <amstan@chromium.org>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>