hw/arm: imx: Don't enable PSCI conduit when booting guest in EL3

Change the iMX-SoC based boards to use the new boot.c functionality
to allow us to enable psci-conduit only if the guest is being booted
in EL1 or EL2, so that if the user runs guest EL3 firmware code our
PSCI emulation doesn't get in its way.

To do this we stop setting the psci-conduit property on the CPU
objects in the SoC code, and instead set the psci_conduit field in
the arm_boot_info struct to tell the common boot loader code that
we'd like PSCI if the guest is starting at an EL that it makes
sense with.

This affects the mcimx6ul-evk and mcimx7d-sabre boards.

Note that for the mcimx7d board, this means that when running guest
code at EL3 there is currently no way to power on the secondary CPUs,
because we do not currently have a model of the system reset
controller module which should be used to do that for the imx7 SoC,
only for the imx6 SoC.  (Previously EL3 code which knew it was
running on QEMU could use a PSCI call to do this.) This doesn't
affect the imx6ul-evk board because it is uniprocessor.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220127154639.2090164-5-peter.maydell@linaro.org