Reads the IPI buffer offset from the FW binary.  The information resides
in addr of .ipi_buffer section.

Moves scp_ipi_init() to rproc_ops::parse_fw() phase.  The IPI buffer can
be initialized only if the offset is clear.

To backward compatible to MT8183 SCP, specify the offset in the board
specific mtk_scp_of_data.  Reads the default offset if the firmware
doesn't have it.
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@google.com>
Link: https://lore.kernel.org/r/20201202044609.2501913-1-tzungbi@google.comSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>