usb: f_gsi: Add API to allow selection of LLCC TCM memory

This change adds and exports API which allows composite function
driver (such as qti_gadget) to select LLCC TCM memory for allocating
USB RMNET IN buffers instead of using DDR memory (default).

Change-Id: I68f46a1e09e732841e12b9dac60181db701a51d3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>