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>