ARM: 9273/1: Add hwcap for Speculation Barrier(SB)

Speculation Barrier(FEAT_SB) is a feature present in AArch32 state for
Armv8 and is represented by ISAR6.SB identification register.

This feature denotes the presence of SB instruction and hence adding a
hwcap will enable the userspace to check it before trying to use this
instruction.

This commit adds the ID feature bit detection, and uses elf_hwcap2
accordingly.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>