misc: Add Arm contributions to gem5-20 RELEASE-NOTES.md

misc: Add Arm contributions to gem5-20 RELEASE-NOTES.md

* Arm architectural extensions
* Arm TFA support
* DRAM changes

Change-Id: I434c501ee8413c8cd64af25c2c18eabf45e3ee77
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Wendy Elsasser <wendy.elsasser@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28908
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>