arm64/hbc: Document HWCAP2_HBC

When we added support for FEAT_HBC we added a new hwcap but did not
document that we had done so, add the documentation.
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-v1-1-797d25f06897@kernel.orgSigned-off-by: Will Deacon <will@kernel.org>

Documentation/arch/arm64/elf_hwcaps.rst

@@ -305,6 +305,9 @@ HWCAP2_SMEF16F16
 HWCAP2_MOPS
     Functionality implied by ID_AA64ISAR2_EL1.MOPS == 0b0001.
 
+HWCAP2_HBC
+    Functionality implied by ID_AA64ISAR2_EL1.BC == 0b0001.
+
 4. Unused AT_HWCAP bits
 -----------------------