UPSTREAM: arm64/sysreg: Add missing ExtTrcBuff field definition to ID_AA64DFR0_EL1

Add the ExtTrcBuff field definitions to ID_AA64DFR0_EL1 from
DDI0601 2023-09.

This field isn't used yet. Adding it for completeness and because
it will be used in future patches.

Bug: 278749606
Change-Id: I87db4ba71abb3b0ee5da1c51f9e964dc4ac68ad7
Signed-off-by: Fuad Tabba <tabba@google.com>
(cherry picked from commit 4f101cdc)