arm64: Take into account ID_AA64PFR0_EL1.CSV3

For non-KASLR kernels where the KPTI behaviour has not been overridden
on the command line we can use ID_AA64PFR0_EL1.CSV3 to determine whether
or not we should unmap the kernel whilst running at EL0.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
[ckadabi@codeaurora.org: Resolve minor merge conflcits]

Change-Id: I49e80ebf4a5daca912758529aaff4d9cd2c178e9
Patch-mainline: linux-arm-kernel @ 05/01/18, 5:12
[ckadabi@codeaurora.org: Use read_system_reg api for reading PFR0_EL1
register and change the arguments to macro ARM64_FTR_BITS as 4.9 kernel
does not support visible field]
Signed-off-by: Channagoud Kadabi <ckadabi@codeaurora.org>