drivers: eth: gmac: Fix cache support for SAM GMAC

What needs to be done for the cache to work properly:
* Make sure cache operations are aligned to 32B
* Make sure to clean and invalidate the operations on gmac descriptors
  (thus all the helper functions)

This commit is needed for SAM GMAC to work when caches are enabled and
MPU mapping is changed to cacheable (See #8185)

Signed-off-by: Tomasz Gorochowik <tgorochowik@antmicro.com>