arm64: docs: Document SSBS HWCAP

We advertise the MRS/MSR instructions for toggling SSBS at EL0 using an
HWCAP, so document it along with the others.
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

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.