arch: arm: cortex_a_r: Add support to start in HYP mode

The ARMv8-R processors always boot into Hyp mode (EL2)

To enter EL1:
Program the HACTLR register because it defaults
to only allowing EL2 accesses. HACTLR controls
whether EL1 can access memory region registers and CPUACTLR.
Program the SPSR before entering EL1.
Other registers default to allowing accesses at EL1 from reset.
Set VBAR to the correct location for the vector table.
Set ELR to point to the entry point of the EL1 code and call ERET.

Signed-off-by: Julien Massot <julien.massot@iot.bzh>