soc: qcom: ipcc: Add the macro IPCC_MPROC_SIGNAL_PING

Introduce the macro IPCC_MPROC_SIGNAL_PING which is used as
the signal-id to ping other subsystem clients. This is mainly
used for testing purposes.

Change-Id: I3482324faebefb9845712acdb727f521afd309eb
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>