target-arm: A64: Generalize ERET to various ELs

Adds support for ERET to and from AArch64 EL2 and 3.

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1400980132-25949-20-git-send-email-edgar.iglesias@gmail.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>