armv8: Handle EL2 Host mode

On implementations that support VHE, the layout of the CPTR_EL2
register depends on whether HCR_EL2.E2H is set.  If the bit is
set, CPTR_EL2 uses the same layout as CPACR_EL1 and can in fact
be accessed through that register.  In that case, jump to the
EL1 code to enable access to the FP/SIMD registers.  This allows
U-Boot to run on systems that pass control to U-Boot in EL2 with
EL2 Host mode enabled such as machines using Apple's M1 SoC.

Signed-off-by: Mark Kettenis <kettenis@openbsd.org>
Acked-by: Marc Zyngier <maz@kernel.org>