ARM: dts: imx: fix clocks for cspi

The CSPI controller has only one clock, but the driver spi-imx.c needs
clock "per" to calculate bitrate divisor.

Signed-off-by: Jonas Andersson <jonas@microbit.se>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>