llcc: Overcome unused function warnings when CONFIG_QCOM_LLCC is not enable

When CONFIG_QCOM_LLCC is not enabled, there is unused function warnings
seen when llcc-tcm.h is added part of client driver.

warning: unused function 'llcc_tcm_get_phys_addr' [-Wunused-function]

Suppress those warnings by adding __maybe_unused attributes with those
APIs.

Change-Id: Ic933b67f95a0f7b2b5577d7d0f79c19cc1dbeb2a
Signed-off-by: Mayank Rana <mrana@codeaurora.org>