remoteproc: imx_rproc: fix build error without CONFIG_MAILBOX

Fix build error when CONFIG_MAILBOX is not set:

arm-linux-gnueabi-ld: drivers/remoteproc/imx_rproc.o: in function `imx_rproc_kick':
imx_rproc.c:(.text+0x328): undefined reference to `mbox_send_message'
arm-linux-gnueabi-ld: drivers/remoteproc/imx_rproc.o: in function `imx_rproc_probe':
imx_rproc.c:(.text+0x52c): undefined reference to `mbox_request_channel_byname'
arm-linux-gnueabi-ld: imx_rproc.c:(.text+0x548): undefined reference to `mbox_request_channel_byname'
arm-linux-gnueabi-ld: imx_rproc.c:(.text+0x76c): undefined reference to `mbox_free_channel'
arm-linux-gnueabi-ld: imx_rproc.c:(.text+0x774): undefined reference to `mbox_free_channel'
arm-linux-gnueabi-ld: imx_rproc.c:(.text+0x7c4): undefined reference to `mbox_free_channel'
arm-linux-gnueabi-ld: drivers/remoteproc/imx_rproc.o: in function `imx_rproc_remove':
imx_rproc.c:(.text+0x86c): undefined reference to `mbox_free_channel'
arm-linux-gnueabi-ld: imx_rproc.c:(.text+0x874): undefined reference to `mbox_free_channel'
make: *** [Makefile:1292: vmlinux] Error 1

Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Fixes: 2df70620

 ("remoteproc: imx_proc: enable virtio/mailbox")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20210331122709.3935521-1-weiyongjun1@huawei.com
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>