ARM: opcodes: add __ERET/__MSR_ELR_HYP instruction encoding

Enabling boot from HYP mode requires the use of some more
virt-specific instructions ("eret" and "msr elr_hyp, reg").

Add the necessary encoding to asm/opcode-virt.h.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>