firmware: qcom: scm: Add function stubs for ARM64

This patch adds stubs for the SCM functions exposed in the QCOM SCM API.

Signed-off-by: Andy Gross <agross@codeaurora.org>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>