cmd_time: add time command

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

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

time: 0.004 seconds, 4 ticks
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Signed-off-by: Che-Liang Chiou <clchiou@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>