README: Fix supported i.MX SoC list for CONFIG_MXC_SPI

CONFIG_MXC_SPI currently works on MX31/35/51 boards, so update the README file.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
diff --git a/README b/README
index caebc22..73ca042 100644
--- a/README
+++ b/README
@@ -1904,7 +1904,7 @@
 		CONFIG_MXC_SPI
 
 		Enables the driver for the SPI controllers on i.MX and MXC
-		SoCs. Currently only i.MX31 is supported.
+		SoCs. Currently i.MX31/35/51 are supported.
 
 - FPGA Support: CONFIG_FPGA