README: Drop README.imx31

The only content of this file is CONFIG options which are no-longer present
in U-Boot. Drop it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: York Sun <york.sun@nxp.com>

doc/README.imx31

@@ -1,29 +0,0 @@
-U-Boot for Freescale i.MX31
-
-This file contains information for the port of U-Boot to the Freescale
-i.MX31 SoC.
-
-1. CONFIGURATION OPTIONS/SETTINGS
----------------------------------
-
-1.1 Configuration of MC13783 SPI bus
-------------------------------------
-	The power management companion chip MC13783 is connected to the
-	i.MX31 via an SPI bus. Use the following configuration options
-	to setup the bus and chip select used for a particular board.
-
-	CONFIG_MC13783_SPI_BUS	-- defines the SPI bus the MC13783 is connected to.
-			   Note that 0 is CSPI1, 1 is CSPI2 and 2 is CSPI3.
-	CONFIG_MC13783_SPI_CS	-- define the chip select the MC13783 s connected to.
-
-1.2 Timer precision
--------------------
-	CONFIG_MX31_TIMER_HIGH_PRECISION
-
-	Enable higher precision timer. The low-precision timer
-	(default) provides approximately 4% error, whereas the
-	high-precision timer is about 0.4% accurate. The extra
-	accuracy is achieved at the cost of higher computational
-	overhead, which, in places where time is measured, should
-	not be critical, so, it should be safe to enable this
-	option.