helpers: Define MIN()

We already have MAX() defined, add MIN() to the common helpers header.

Using the common helper also fixes a bug in tpmdrivers's MIN() where
it was reverted.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
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Changes:
v2:
* updated the comment about a fixed bug