firmware: imx: Align imx_sc_msg_req_cpu_start to 4

The imx SC api strongly assumes that messages are composed out of
4-bytes words but some of our message structs have odd sizeofs.

This produces many oopses with CONFIG_KASAN=y.

Fix by marking with __aligned(4).

Fixes: d90bf296 ("firmware: imx: Add support to start/stop a CPU")
Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>