coresight: byte-cntr: Change minimal block size to 4096

Too small block size like 16 has too many interrupts under
high data rate. It creates big pressue on system.  Change
minimal block size of byte counter interrupt to 4096 byte,
so interrupt will be raised with minimal 4096 byte data.

Change-Id: I3c1b14ef58a1b783f70cd15c748832e431b97296
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>