edgetpu: use kvmalloc for pages array

Under heavy memory pressure, allocating a physically contiguous buffer
for a struct pages * array may fail. Since this array does not need to
be physically contiguous, switch to kvmalloc instead.

Bug: 193759145

Signed-off-by: Erick Reyes <erickreyes@google.com>
Change-Id: I810019402315e7211e00f942dff5fc32687a3d25