Add i.MX7 SRC device implementation

The SRC device is normally used to start the secondary CPU.

When running Linux directly, QEMU is emulating a PSCI interface that UBOOT
is installing at boot time and therefore the fact that the SRC device is
unimplemented is hidden as Qemu respond directly to PSCI requets without
using the SRC device.

But if you try to run a more bare metal application (maybe uboot itself),
then it is not possible to start the secondary CPU as the SRC is an
unimplemented device.

This patch adds the ability to start the secondary CPU through the SRC
device so that you can use this feature in bare metal applications.

Signed-off-by: Jean-Christophe Dubois <jcd@tribudubois.net>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: ce9a0162defd2acee5dc7f8a674743de0cded569.1692964892.git.jcd@tribudubois.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>