Add a dhrystone benchmark command

Drystone provides a convenient sanity check that the CPU is running at full
speed. Add this as a command which can be enabled as needed.

Note: I investigated using

Add a dhrystone benchmark command

Drystone provides a convenient sanity check that the CPU is running at full
speed. Add this as a command which can be enabled as needed.

Note: I investigated using Coremark for this but there was a license
agreement and I could not work out if it was GPL-compatible.

Signed-off-by: Simon Glass <sjg@chromium.org>

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