drivers: scmi: scmi_sq: Modify wrong payload length

Payload length of the get dram mapping information message is 0.
The mbx_mem->len parameter should be 4, it only contains
message header.

Fixes: b67d2029 ("plat/synquacer: enable SCMI support")
Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
Change-Id: If1cd4c855da2dc5dc4b6da3bea152b8441971de7