hw/arm: make cpu targeted by arm_load_kernel the primary CPU.

Currently, arm booting processus assumes that the first_cpu is the CPU
that will boot: `arm_load_kernel` is powering off all but the `first_cpu`;
`do_cpu_reset` is setting the loader address only for this `first_cpu`.

For most of the boards, this isn't an issue as the kernel is loaded and
booted on the first CPU anyway. However, for zynqmp, the option
"boot-cpu" allows to choose any CPUs.

Create a new arm_boot_info entry `primary_cpu` recording which CPU will
be boot first. This one is set when `arm_boot_kernel` is called.

Signed-off-by: Clément Chigot <chigot@adacore.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20250526085523.809003-2-chigot@adacore.com
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

include/hw/arm/boot.h

@@ -132,6 +132,9 @@ struct arm_boot_info {
     bool secure_board_setup;
 
     arm_endianness endianness;
+
+    /* CPU having load the kernel and that should be the first to boot.  */
+    ARMCPU *primary_cpu;
 };
 
 /**