arch-arm: Add E2H bit to HCR_EL2 System register

arch-arm: Add E2H bit to HCR_EL2 System register

This patch adds the EL2 Host bit to the HCR_EL2 register.  Enables a
configuration where a Host Operating System is running in EL2, and the
Host Operating System's applications are running in EL0.

Change-Id: I92d21ed9f8958c58f135dca1b6a97460ba4c02f9
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/10381
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>