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 <8eef13a4f970e2e9372c8d26ad0737b760f99c02@arm.com>
Reviewed-by: Nikos Nikoleris <b6a1b3cd9d36c44e9eeb16bc870df3cb2a0cf469@arm.com>
Reviewed-by: Wendy Elsasser <7a56f97b62bb96436d58310e53b4574a89e22a06@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28908
Reviewed-by: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Maintainer: Jason Lowe-Power <92de577f37050ad05b60626d2f17913f40dcb1a2@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5