soc: qcom: ipa: GSI headers

The Generic Software Interface is a layer of the IPA driver that
abstracts the underlying hardware.  The next patch includes the
main code for GSI (including some additional documentation).  This
patch just includes three GSI header files.

  - "gsi.h" is the top-level GSI header file.  This structure is
    is embedded within the IPA structure.  The main abstraction
    implemented by the GSI code is the channel, and this header
    exposes several operations that can be performed on a GSI channel.

  - "gsi_private.h" exposes some definitions that are intended to be
    private, used only by the main GSI code and the GSI transaction
    code (defined in an upcoming patch).

  - Like "ipa_reg.h", "gsi_reg.h" defines the offsets of the 32-bit
    registers used by the GSI layer, along with masks that define the
    position and width of fields less than 32 bits located within
    these registers.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>