spi/imx: add support for imx51's eCSPI and CSPI

i.MX51 comes with two eCSPI interfaces (that are quite different from
what was known before---the tried and tested Freescale way) and a CSPI
interface that is identical to the devices found on i.MX25 and i.MX35.

This patch is a merge of two very similar patches (by Jason Wang and Sascha
Hauer resp.) plus a (now hopefully correct) reimplementation of the
clock calculation.

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