coresight: csr: update programming sequence of enabling etr to bam

There is a new programming sequence on Kona, the register of
QDSS_QDSS_CS_QDSSCSR_USBFLSHCTRL should be configured after
programming TMC ETR registers. This changes use new programming
sequence to enable etr to bam. Add sysfs interface in CSR to let
user space change usb flush period.

Change-Id: I0cee7db746106fd0318d01a92682f185cb31ad97
Signed-off-by: Yuanfang Zhang <zhangyuanfang@codeaurora.org>