cmd_time: add time command

The 'time' command runs and reports execution time of commands.

Sample usage:
--------------------
u-boot# time crc 0x1000 1000
CRC32 for 00001000 ... 00001fff ==> ae94dc4b

time: 0.004 seconds, 4 ticks
--------------------

Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>
diff --git a/README b/README
index c5cfb13..3a8938f 100644
--- a/README
+++ b/README
@@ -783,6 +783,7 @@
 		CONFIG_CMD_SOURCE	  "source" command Support
 		CONFIG_CMD_SPI		* SPI serial bus support
 		CONFIG_CMD_TFTPSRV	* TFTP transfer in server mode
+		CONFIG_CMD_TIME		* run command and report execution time
 		CONFIG_CMD_USB		* USB support
 		CONFIG_CMD_CDP		* Cisco Discover Protocol support
 		CONFIG_CMD_FSL		* Microblaze FSL support