i.MX31: document timer precision option

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

diff --git a/README.imx31 b/README.imx31
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+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.