spi/imx: Support different fifo sizes

The i.MX51 ECSPI has a fifo size of 64 entries instead of 8 entries as
found on the other cspi bus devices.

Cc: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: David Jander <david@protonic.nl>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>