hw/arm/boot: Drop nb_cpus field from arm_boot_info

hw/arm/boot: Drop nb_cpus field from arm_boot_info

We use the arm_boot_info::nb_cpus field in only one place, and that
place can easily get the number of CPUs locally rather than relying
on the board code to have set the field correctly.  (At least one
board, xlnx-versal-virt, does not set the field despite having more
than one CPU.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20220127154639.2090164-16-peter.maydell@linaro.org