neoverse-rd: define system counter impdef registers

The system counter implementation specific registers are similar for
RD-N2 and RD-Fremont platforms. So add common file that defines the
impdef register offsets in the system counter register frame.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I5424f5d36c3099ec3f8a7876cdb697c3affeac8c