hw/arm/boot: If booting a kernel in EL2, set SCR_EL3.HCE

hw/arm/boot: If booting a kernel in EL2, set SCR_EL3.HCE

If we're directly booting a Linux kernel and the CPU supports both
EL3 and EL2, we start the kernel in EL2, as it expects. We must also
set the SCR_EL3.HCE bit in this situation, so that the HVC
instruction is enabled rather than UNDEFing. Otherwise at least some
kernels will panic when trying to initialize KVM in the guest.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20180313153458.26822-4-peter.maydell@linaro.org