defconfig: sdxpoorwills: Enable mailbox driver

Temporarily enable mailbox driver as PIL driver requires mailbox
APIs. This defconfig will be automatically enabled later when QTI_RPMH_API
is enabled.

Change-Id: I37241f409f214f7440f566a9158ec7eaf840d604
Signed-off-by: Kyle Yan <kyan@codeaurora.org>