common/octeontx2: fix mbox memory access

common/octeontx2: fix mbox memory access

Octeontx2 PMD's mailbox client uses device memory to send messages
to mailbox server in the admin function Linux kernel driver.
The device memory used for the mailbox communication needs to
be qualified as volatile memory type to avoid unaligned device
memory accesses because of compiler's memory access coalescing.

This patch modifies the mailbox request and responses as volatile
type which were non-volatile earlier and accessed from unaligned
memory addresses which resulted in bus errors on Fedora 30 with
gcc 9.1.1.

Fixes: 2b71657c8660 ("common/octeontx2: add mbox request and response definition")

Signed-off-by: Vamsi Attunuru <vattunuru@marvell.com>
Signed-off-by: Nithin Dabilpuram <ndabilpuram@marvell.com>
Acked-by: Jerin Jacob <jerinj@marvell.com>