platform-drivers: msm: sps: add stubs for SPS APIs

Some client drivers exist on old targets where BAM HW and SPS driver
do not exist. Add stubs for SPS APIs so that such client drivers can
be compiled successfully for old targets as well as new targets.

Change-Id: I2f478c91e048a4e387fe681236258305795c00d5
Signed-off-by: Yan He <yanhe@codeaurora.org>