GSI firmware for IPA must be loaded during initialization, either by
the AP or by the modem.  The loader is currently specified based on
whether the Boolean modem-init property is present.

Instead, use a new property with an enumerated value to indicate
explicitly how GSI firmware gets loaded.  With this in place, a
third approach can be added in an upcoming patch.

The new qcom,gsi-loader property has two defined values:
  - self:   The AP loads GSI firmware
  - modem:  The modem loads GSI firmware
The modem-init property must still be supported, but is now marked
deprecated.

Update the example so it represents the SC7180 SoC, and provide
examples for the qcom,gsi-loader, memory-region, and firmware-name
properties.
Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>