spi: Introduce one new field to set word delay

For some SPI controllers, after each word size (specified by bits_per_word)
transimission, the hardware need some delay to make sure the slave has enough
time to receive the whole data.

So introducing one new 'word_delay' field of struct spi_tansfer for slave
devices to set this inter word delay time.

Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>