arm64: add midr_el1 accessor function

arm64: add midr_el1 accessor function

Provide access to the MIDR_EL1 register to obtain the
main id for determining CPU implementer and part/revision
information.

BUG=chrome-os-partner:31761
BRANCH=None
TEST=Built and printed the output of this function on ryu.

Change-Id: I42cec75072fc5e8b48f63c1971840fdc415e4326
Signed-off-by: Patrick Georgi <pgeorgi@chromium.org>
Original-Commit-Id: ad19ffe629d9f16b8fd07051ce73533e97fb3f5c
Original-Change-Id: I8b8506ebff8e6f9d7c4f96d7ff7e21803972961e
Original-Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/216423
Original-Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-on: http://review.coreboot.org/9032
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>