drm/mali-dp: Add core_id file to the sysfs interface

Add a core_id file in the driver's sysfs directory,
exposing the hardware CORE ID. This is useful to allow
userspace to discover the hardware version used.

Signed-off-by: Mihail Atanassov <mihail.atanassov@arm.com>
Signed-off-by: Liviu Dudau <Liviu.Dudau@arm.com>