Currently arm_pmu_device_probe contains probing logic specific to the
platform_device infrastructure, and some logic required to safely
register the PMU with various systems.

This patch factors out the logic relating to the registration of the
PMU. This makes arm_pmu_device_probe a little easier to read, and will
make it easier to reuse the logic for an ACPI-specific probing
mechanism.
Signed-off-by: NMark Rutland <mark.rutland@arm.com>
Tested-by: NJeremy Linton <jeremy.linton@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NWill Deacon <will.deacon@arm.com>