firmware: imx: scu: increase RPC timeout

When system loading is high, we can meet some command timeout
issue occasionally, so increase the timeout to a safe value.

Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>