coresight: Add support for TMC ETR SG unit

This patch adds support for setting up an SG table used by the
TMC ETR inbuilt SG unit. The TMC ETR uses 4K page sized tables
to hold pointers to the 4K data pages with the last entry in a
table pointing to the next table with the entries, by kind of
chaining. The 2 LSBs determine the type of the table entry, to
one of :

 Normal - Points to a 4KB data page.
 Last   - Points to a 4KB data page, but is the last entry in the
          page table.
 Link   - Points to another 4KB table page with pointers to data.

The code takes care of handling the system page size which could
be different than 4K. So we could end up putting multiple ETR
SG tables in a single system page, vice versa for the data pages.

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>