i.MX31: document timer precision option

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>

README.imx31

+i.MX31 specific Configuration Options:
+--------------------------------------
+
+- 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.