Fix completion descriptors alignment for the ENA

The latest generation hardware requires IO CQ (completion queue)
descriptors memory to be aligned to a 4K. It needs that feature for
the best performance.

Allocating unaligned descriptors will have a big performance impact as
the packet processing in a HW won't be optimized properly. For that
purpose adjust ena_dma_alloc() to support it.

It's a critical fix, especially for the arm64 EC2 instances.

Submitted by: Ido Segev <idose@amazon.com>
Obtained from: Amazon, Inc
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D27114