firmware: arm_scmi/perf: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertenly introduced[3] to the codebase from now on.

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732 ("cxgb3/l2t: Fix undefined behaviour")

Replace the zero-length member "opp" in scmi_msg_resp_perf_describe_levels
structure with flexible-array.

Link: https://lore.kernel.org/r/20200211231252.GA14830@embeddedorSigned-off-by: NGustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/perf.c

@@ -89,7 +89,7 @@ struct scmi_msg_resp_perf_describe_levels {
 		__le32 power;
 		__le16 transition_latency_us;
 		__le16 reserved;
-	} opp[0];
+	} opp[];
 };
 
 struct scmi_perf_get_fc_info {