Don't allow pooling ops with excessive padding

Padding larger than the kernel size leads to the
odd effect where no values from the input tensor are used.
This leads to a divide by 0 in AVG_POOL2D, as it only divides
by the number of valid input values used, not padding.
Change MAX_POOL2D to have the same limitation for consistency.

Signed-off-by: Eric Kunze <eric.kunze@arm.com>
Change-Id: I9cd0c0619f88f570d30c83053280460ae3d1cbfb