Introduce syscounter_read() helper function

syscounter_read() reads the system counter value. It encapsulates the
instruction barrier required to get an accurate timestamp for any
time-sensitive related operation.

Change-Id: Ib2cfe7789ee1471e614140d425d29ab5c8d306ea
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>