net: ftgmac100: Modify desc. size to cache line

net: ftgmac100: Modify desc. size to cache line

The TX/RX descriptor size is 16 byte.
When the cache line size is larger than 16 bytes, descriptors
flushed to RAM will flush more than one descriptor.
It is possible that it may mistakenly flush to other descriptor
that has been updated by MAC in RAM.

To avoid this issue, align the descriptors to cache line size.
Only one desc will be flushed or invalidated at a time.

Signed-off-by: Jacky Chou <jacky_chou@aspeedtech.com>
Acked-by: Leo Yu-Chi Liang <ycliang@andestech.com>