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.
Signed-off-by: Fuad Tabba <tabba@google.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20231214100158.2305400-3-tabba@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/tools/sysreg

@@ -1189,7 +1189,10 @@ Enum	63:60	HPMN0
 	0b0000	UNPREDICTABLE
 	0b0001	DEF
 EndEnum
-Res0	59:56
+UnsignedEnum	59:56	ExtTrcBuff
+	0b0000	NI
+	0b0001	IMP
+EndEnum
 UnsignedEnum	55:52	BRBE
 	0b0000	NI
 	0b0001	IMP