arm64: ptrauth: Add bootup/runtime flags for __cpu_setup

This patch allows __cpu_setup to be invoked with one of these flags,
ARM64_CPU_BOOT_PRIMARY, ARM64_CPU_BOOT_SECONDARY or ARM64_CPU_RUNTIME.
This is required as some cpufeatures need different handling during
different scenarios.

The input parameter in x0 is preserved till the end to be used inside
this function.

There should be no functional change with this patch and is useful
for the subsequent ptrauth patch which utilizes it. Some upcoming
arm cpufeatures can also utilize these flags.

Suggested-by: James Morse <james.morse@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Amit Daniel Kachhap <amit.kachhap@arm.com>