drivers/octeontx2: fix icc build for i686

drivers/octeontx2: fix icc build for i686

Fix the following ICC specific compilation issue with i686 build.

dpdk/drivers/common/octeontx2/otx2_mbox.c(47): error #2259:
non-pointer conversion from "unsigned long long" to "struct mbox_hdr *"
may lose significant bits
(struct mbox_hdr *)((uintptr_t)mdev->mbase + mbox->tx_start);

Fixes: 732377a6792e ("mempool/octeontx2: add build infra and device probe")
Fixes: 371d3212cbed ("common/octeontx2: add build infrastructure and HW definition")

Signed-off-by: Amit Gupta <agupta3@marvell.com>