coresight:byte-cntr: fix the byte-counter of ETR

When ETR byte-cntr is enabled, ETR triggers an interrupt at
ETRBYTECNT. It indicates atleast 'block_size' bytes of data
is captured in DDR. The Byte-cntr is a 8-byte counter, so
the CSR register ETRBYTECNT is set to 'block_size/8'.

Change-Id: I62871b4a62216f35365ee490f6b0946a039e182e
Signed-off-by: Saranya Chidura <schidura@codeaurora.org>