Using the opcode directly is necessary for the transition to the GCC
6.1.0 based toolchain, because the old toolchain only supports eret and
the new toolchain only supports mret.
Change-Id: I17e14d4793ae5259f7ce3ce0211cbb27305506cc
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/15290
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>