arm64: cpufeature: Track 32bit EL0 support

Add cpu_hwcap bit for keeping track of the support for 32bit EL0.

Tested-by: Yury Norov <ynorov@caviumnetworks.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>