firmware: qcom: scm: Add empty functions to help compile testing

This will help to compile testing drivers which depends on scm
functions with COMPILE_TEST Kconfig option.

Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Reviewed-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>