ARM: 9274/1: Add hwcap for Speculative Store Bypassing Safe

Speculative Store Bypassing Safe(FEAT_SSBS) is a feature present in
AArch32 state for Armv8 and is represented by ID_PFR2_EL1.SSBS
identification register.

This feature denotes the presence of PSTATE.ssbs bit and hence adding a
hwcap will enable the userspace to check it before trying to set/unset
this PSTATE.

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>