coresight: tmc: Fix write point for ETR sg mode

Write point of ETR could be over 4GB on device with more
than 4GB memory.  Align the write point to phys_addr_t
type to fix write point overflow issue.

Change-Id: I0cd4aca2214b090101762ccc7a0ed1534f6dff11
Signed-off-by: Tingwei Zhang <tingwei@codeaurora.org>