spi, spi_mxc: do not hang in spi_xchg_single

if status register do never set MXC_CSPICTRL_TC, spi_xchg_single
endless loops. Add a timeout here to prevent endless hang.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Dirk Behme <dirk.behme@gmail.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
diff --git a/README b/README
index f704eb3..c52ffd9 100644
--- a/README
+++ b/README
@@ -2597,6 +2597,10 @@
 		Enables the driver for the SPI controllers on i.MX and MXC
 		SoCs. Currently i.MX31/35/51 are supported.
 
+		CONFIG_SYS_SPI_MXC_WAIT
+		Timeout for waiting until spi transfer completed.
+		default: (CONFIG_SYS_HZ/100)     /* 10 ms */
+
 - FPGA Support: CONFIG_FPGA
 
 		Enables FPGA subsystem.