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

firmware: arm_scmi: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Replace the zero-length member "msg_payload" in scmi_shared_mem
structure with flexible-array.

Link: https://lore.kernel.org/r/20200211231045.GA13956@embeddedor
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
[ rebased the change as files are moved around ]
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>