ARM: qcom_defconfig: Enable MAILBOX

Problem:
ab460a2e ("rpmsg: qcom_smd: Access APCS through mailbox framework"
added a "depends on MAILBOX") to RPMSG_QCOM_SMD, thus RPMSG_QCOM_SMD
becomes unset since MAILBOX was not enabled in qcom_defconfig and is
not otherwise selected for the dragonboard.  When the resulting
kernel is booted the mmc device which contains the root file system
is not available.

Fix:
add CONFIG_MAILBOX to qcom_defconfig

Fixes: ab460a2e ("rpmsg: qcom_smd: Access APCS through mailbox framework"
added a "depends on MAILBOX")

Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>