arm64: Document missing userspace visible fields in ID_AA64ISAR2_EL1

We have exposed a number of fields in ID_AA64ISAR2_EL1 to userspace without
adding the matching documentation in cpu-feature-registers.rst, update it
to match the implementation.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Joey Gouly <joey.gouly@arm.com>
Link: https://lore.kernel.org/r/20230914-arm64-feat-hbc-doc-v1-2-797d25f06897@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: broonie

Documentation/arch/arm64/cpu-feature-registers.rst

@@ -290,8 +290,18 @@ infrastructure:
      +------------------------------+---------+---------+
      | Name                         |  bits   | visible |
      +------------------------------+---------+---------+
+     | CSSC                         | [55-52] |    y    |
+     +------------------------------+---------+---------+
+     | RPRFM                        | [51-48] |    y    |
+     +------------------------------+---------+---------+
+     | BC                           | [23-20] |    y    |
+     +------------------------------+---------+---------+
      | MOPS                         | [19-16] |    y    |
      +------------------------------+---------+---------+
+     | APA3                         | [15-12] |    y    |
+     +------------------------------+---------+---------+
+     | GPA3                         | [11-8]  |    y    |
+     +------------------------------+---------+---------+
      | RPRES                        | [7-4]   |    y    |
      +------------------------------+---------+---------+
      | WFXT                         | [3-0]   |    y    |