docs/arm64: cpu-feature-registers: Documents missing visible fields

A couple of fields visible to userspace are not described in the
documentation. So update it.
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Documentation/arm64/cpu-feature-registers.rst

@@ -193,6 +193,10 @@ infrastructure:
      +------------------------------+---------+---------+
      | Name                         |  bits   | visible |
      +------------------------------+---------+---------+
+     | SB                           | [36-39] |    y    |
+     +------------------------------+---------+---------+
+     | FRINTTS                      | [32-35] |    y    |
+     +------------------------------+---------+---------+
      | GPI                          | [31-28] |    y    |
      +------------------------------+---------+---------+
      | GPA                          | [27-24] |    y    |