coresight: byte-cntr: Read 64bit rwp

TMC RWP has two 32bit registers which combine to one 64bit register.
Read correct RWP register in byte counter driver.

Change-Id: If4a204110e0cc97d65e8a6af37ca764ffb38a150
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>