docs/arm64: elf_hwcaps: Document HWCAP_SB

All the hardware capabilities but HWCAP_SB is not documented in
elf_hwcaps.rst. So document 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/elf_hwcaps.rst

@@ -155,6 +155,9 @@ HWCAP_FLAGM
 HWCAP_SSBS
     Functionality implied by ID_AA64PFR1_EL1.SSBS == 0b0010.
 
+HWCAP_SB
+    Functionality implied by ID_AA64ISAR1_EL1.SB == 0b0001.
+
 HWCAP_PACA
     Functionality implied by ID_AA64ISAR1_EL1.APA == 0b0001 or
     ID_AA64ISAR1_EL1.API == 0b0001, as described by