of: Fix iteration bug over CPU reg properties

The size of each hwid in a cpu nodes 'reg' property is defined by the
parents #address-cells property in the normal way. The cpu parsing code
has a bug where it will overrun the end of the property if
address-cells is greater than one. This commit fixes the problem by
adjusting the array size by the number of address cells. It also makes
sure address-cells isn't zero for that would cause an infinite loop.

v2: bail if #address-cells is zero instead of forcing to
    OF_ROOT_NODE_ADDR_CELLS_DEFAULT. Forcing it will cause the reg
    property to be parsed incorrectly.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>