UPSTREAM: coresight: tmc: Hide trace buffer handling for file read

At the moment we adjust the buffer pointers for reading the trace
data via misc device in the common code for ETF/ETB and ETR. Since
we are going to change how we manage the buffer for ETR, let us
move the buffer manipulation to the respective driver files, hiding
it from the common code. We do so by adding type specific helpers
for finding the length of data and the pointer to the buffer,
for a given length at a file position.

Cc: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

(Upstream commit 3495722a0093c80f66c020f90161983b46a1f5ee).

Bug: 140266694
Change-Id: If57b9ea693ff42075ae8f39adf1c3ff003bd3128
Signed-off-by: Yabin Cui <yabinc@google.com>