coresight: stm: Move ACPI device support from AMBA to platform driver

This drops cpu debug from the AMBA ACPI device list and instead moves them
inside platform ACPI devices list which can be detected, and processed via
a new platform driver i.e stm_platform_driver.

A new helper __debug_probe() which has been refactored from debug_probe(),
which now can be used from both AMBA and platfrom driver probe functions.

Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>