arm64: docs: Document SSBS HWCAP

[ Upstream commit ee91176120bd584aa10c564e7e9fdcaf397190a1 ]

We advertise the MRS/MSR instructions for toggling SSBS at EL0 using an
HWCAP, so document it along with the others.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/arm64/elf_hwcaps.txt

@@ -178,3 +178,7 @@ HWCAP_ILRCPC
 HWCAP_FLAGM
 
     Functionality implied by ID_AA64ISAR0_EL1.TS == 0b0001.
+
+HWCAP_SSBS
+
+    Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.