hw/arm/boot: Prevent setting both psci_conduit and secure_board_setup

hw/arm/boot: Prevent setting both psci_conduit and secure_board_setup

Now that we have dealt with the one special case (highbank) that needed
to set both psci_conduit and secure_board_setup, we don't need to
allow that combination any more. It doesn't make sense in general,
so use an assertion to ensure we don't add new boards that do it
by accident without thinking through the consequences.

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: 20220127154639.2090164-13-peter.maydell@linaro.org