The global processor_id is assigned the MIDR_EL1 value of the boot
CPU in the early init code, but is never referenced afterwards.

As the relevance of the MIDR_EL1 value of the boot CPU is debatable
anyway, especially under big.LITTLE, let's remove it before anyone
starts using it.
Tested-by: NMark Rutland <mark.rutland@arm.com>
Reviewed-by: NMark Rutland <mark.rutland@arm.com>
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NWill Deacon <will.deacon@arm.com>