remoteproc: qcom: q6v5-mss: Fix build error without QCOM_MDT_LOADER

If QCOM_Q6V5_MSS is set but QCOM_MDT_LOADER is not,
building will fails:

drivers/remoteproc/qcom_q6v5_mss.o: In function `q6v5_start':
qcom_q6v5_mss.c:(.text+0x3260): undefined reference to `qcom_mdt_read_metadata'

Add QCOM_MDT_LOADER dependency for QCOM_Q6V5_MSS.

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: f04b9138

 ("remoteproc: qcom: q6v5-mss: Support loading non-split images")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>