arm64: record boot mode when entering the kernel

To be able to signal the availability of EL2 to other parts of
the kernel, record the boot mode.

Once booted, two predicates indicate if HYP mode is available,
and if not, whether this is due to a boot mode mismatch or not.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>